Add connectors that bind to a shape and follow it

[marcosqlbi]

Pull request 5 of the 1.6.0 design objects plan, for issue #131. It adds the
third thing the Insert row offers: a line, an arrow, or a curved arrow drawn by
dragging, which can bind to a shape or a container and then follows it.

Why it is built the way it is:

• A connector is a board object like any other, and not a container. It
  carries its two ends and, for each, an optional anchor - the object it is tied
  to and where on that object's box, as a fraction each way. The fraction rather
  than the point is what is kept, so an endpoint follows a move, a scale, and a
  stretch without the file recording the same thing twice, and a board opens with
  its connectors where they were drawn without anything being worked out from
  what is around them.
• One description of the geometry. Core.Geometry.ConnectorGeometry gives
  the path as a polyline, the arrowhead, the shortened line under it, and the
  box, so the renderer, the hit test, the area selection, the binding dots, and a
  later vector export cannot disagree about where the line is. A curve is one
  cubic whose control points leave each end along the outward normal of the side
  the anchor sits on, flattened to 32 segments.
• Following happens where linked strokes are already carried. The group
  gesture collects the connectors bound to a selected object that are not
  themselves selected, recomputes them from their anchors against the new bounds,
  and puts them in the same ReplaceObjectsCommand, so one undo restores the
  shapes and the arrows together.
• Deleting a shape frees its arrows rather than taking them. That is two
  changes to the document and one thing that happened, so CompositeCommand
  joins them into one step of the history.
• An export area is never cut between a shape and its arrow.
  BoardPartitioner puts a connector in the unit of what it is bound to, and a
  connector bound at both ends merges those two units.

It affects the board format (a board with a connector is written as version 7,
as one with a shape or a label already is), AppSettings (a Connector block of
last-used colour, thickness, and kind - settings version stays 19), the property
bar (Colour and Thickness now apply to connectors, and a Line kind row appears
when everything selected is one), the Insert row and the optional toolbar flyout,
and the overlay picture in an editable export, which now includes connectors so
none of them vanishes from a deck or a PDF. Native cxnSp and PDF paths are
pull request 6, which is being built in parallel; the change to EditableSlide
here is the smallest one that keeps connectors visible, and nothing in
SQLBI.Whiteboard.Export is touched.

Decisions the plan did not settle

• A free end of a curve leaves horizontally towards the other end. The plan
  says "horizontally when free"; for two ends at the same X that is a tie, and it
  is broken rightwards, which makes a vertical free curve hook round to point
  back at itself. It looks deliberate rather than broken, and every other case
  reads well, so it is left as the plan wrote it.
• The binding reach is 16 screen pixels and the tap band is still 8. A drop
  is not a tap: the endpoint is already where the hand put it, and the only
  question is whether it meant the object. Decision 6 gives the 16; the hit test
  keeps BoardObject.HitBand.
• With Ctrl the target still has to be under the pointer, but the nearest
  border point may be any distance away, so there is no reach test on that path.
  A shape answers with its own outline, everything else with its box.
• A lone connector has no corner handle at all - not the drawn one, not the
  resize cursor, and not the gesture. Its box is a box around a line, so what it
  offers instead is its two ends.
• Dragging a connector by its body detaches both ends, as the plan says, and
  that is recorded as part of the same move, so one undo puts the bindings back.
• The Insert row now has two buttons labelled Arrow - the block arrow shape
  and the arrow connector - on either side of a separator. Both names come from
  the plan, and the tooltips say which is which.
• The connector glyphs are hand-drawn on the 24-unit grid. Fluent has arrows,
  but none of them is a line between two things, and the shape glyphs beside them
  are hand-drawn for the same reason.
• Under Mouse drawing off, a left drag still borrows Select, so a connector
  is drawn with a mouse only when mouse drawing is on. That is exactly how the
  shape tool behaves, and changing it is not this pull request's to make.

To try by hand

• Draw one of each kind with the pen, with a finger under Finger drawing, and
  with the mouse; check that the live preview follows and that a tap without a
  drag makes nothing.
• Drop an end near a corner and near a side midpoint of a shape, a picture, a
  text container, and a label, and watch the eight dots appear within about a
  finger's width. Hold Ctrl at release and check the end lands on the outline
  itself - on an ellipse and a diamond, where the outline and the box differ most.
• Bind both ends, move and then resize one shape, and check the arrow follows and
  that one undo puts everything back.
• Select the connector alone: drag each endpoint, drag the body, and check that
  the property bar offers colour, thickness, and the three kinds, and that
  applying one changes every selected connector at once.
• Delete a shape that two arrows point at; they should stay where they were, and
  one undo should bring the shape and the bindings back.
• Rubber band and lasso across a curve rather than across its box, under both
  Area selects settings.
• Export a board of shapes joined by connectors to a deck and to a PDF and check
  nothing is missing and that no slide cuts between a shape and its arrow.

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