Draw window controls on GNOME and make text selectable

[fzakaria]

I have really enjoyed this app! I suspect you did it for https://omarchy.us/ + Hyprland but I was enjoying it on https://nixos.org/ + Wayland.

Here are two fixes for running disktree on GNOME (Wayland).

Window controls where the compositor draws none

disktree asks for server-side decorations. Hyprland, KDE, X11 window
managers, macOS and Windows draw a title bar for it. GNOME's Mutter does
not offer server-side decorations to Wayland apps, so GPUI falls back to
client-side ones, and disktree drew nothing: no minimize, maximize or
close, no way to move the window, no resize edges.

When the window reports client-side decorations, each screen's header is
now the title bar:

• drag to move (after a 4 px threshold, so a click stays a click)
• double-click to maximize or restore, right-click for the window menu
• minimize / maximize / close at the right end, limited to what the
  compositor says it supports
• the free edges and corners resize the window

Presses on the header's crumbs and settings never reach the title bar,
so double-clicking a crumb does not maximize the window. Under
server-side decorations nothing changes.

Selectable text and Ctrl+C

Nothing could be selected, so a path in the panel had to be retyped.
This uses gpui-kit's own window-wide text selection (TextSelectionLayer
plus SelectableText). The selection's name and path, the marked and
review lists, the removal log and failures, the disk figures, notices
and the key bar are selectable. Ctrl+C (Cmd+C, with an Edit menu on
macOS) copies; with nothing selected the chord is left alone and is not
read as c.

gpui-kit moves a selection as the pointer drags but leaves the redraw to
the app, so the root refreshes the window during a selection drag;
without it the highlight lagged.

Text stays plain where a press already means something: buttons, crumbs,
clickable rows, the title bar, the cursor tooltip, and the painted
treemap.

Testing

• cargo xtask lint and cargo xtask test pass on each commit.
• New window-harness tests: no chrome and no window move under
  server-side decorations (the test window panics if asked to move);
  controls and resize handles under client-side decorations, standing in
  for GNOME through a test-only field; a drag across the selection's
  name and path followed by Ctrl+C copies exactly that text.
• Unit tests for which controls show, which edges resize, and the move
  threshold.
• Run by hand on GNOME 49 / Wayland (NixOS): move, maximize, minimize,
  close, resize, select and copy. Not tried on Windows or macOS beyond
  CI, or on Hyprland by hand.

Screenshots

The controls in the header on GNOME, and a selection in the panel.