Add multicursor autocompletions

[redsti-github]

Implements autocomplete while using multiple cursors. (loosely based on #3442)

Only autocompletes cursors which end in the same word as the active cursor.

For example:

foo bar
f|     -> foo|
f|     -> foo|
b|     -> b|
f|b    -> f|b

Closes #3442

[JoeKar]

But we already have #3442 and this use case is discussed within it?!
I don't see any reason to duplicate the PR.

[redsti-github]

Yes, but #3442 still autocompletes on all cursors, even if they shouldn't be.
I created this PR because as far as i know, i can't add or suggest commits to someone else's PR. (Atleast not without opening a PR to the author's repo, or commenting a diff, both of which feel impractical to me.)

[Neko-Box-Coder]

Tested the changes, works well as expected. Nice one.

[Neko-Box-Coder]

Hey @redsti-github , I want to merge this to my own fork.

Could you tidy up the commits please?
i.e. "fix plugin support" and "remove trailing whitespace" should be in "Autocomplete support for multiple cursors" instead of having their own commits.

Thanks.

[Neko-Box-Coder]

I have tested this on my own fork for quite awhile and I haven't encountered any issues. Maybe we can merge this @JoeKar @dmaluka ?

[dmaluka]

This caused a serious regression: autocompletion in the command bar doesn't work unless the cursor is after a word.

For example, open the command prompt and press Tab - nothing happens (while it should suggest commands to execute).

Or for example, open the command prompt, type open and press Tab - nothing happens (while it should suggest files to open).

[Neko-Box-Coder]

So there are 2 autocomplete functions (confusingly), one in buffer and one in action, where the action one calls the buffer one.

And there's also CommandComplete, for the command bar, which calls the buffer autocomplete directly.

Previously, the autocomplete in the action one was guarded with some autocomplete logic (is the cursor in words, etc...).

This PR moved the guard to the buffer autocomplete, and now the CommandComplete which wasn't guarded by this logic before is guarded now. Unfortunately I didn't spot this :/ (I have always typed the first few letters before auto-completing...)

I guess the fix would be moving the guard back to the action autocomplete function instead of in the buffer one?

[Andriamanitra]

I also noticed this quirk that can happen when the words under different cursors don't match:

https://asciinema.org/a/mc6ZvLipLNBv1Wmd

This also broke plugins that autocomplete sequences with non-word characters. For example jlabbrev lets you autocomplete \^ to various LaTeX superscript expressions (\^2, \^alpha, \^(, ...). I confirmed that #4200 fixes that issue for single cursor but it won't work with multiple cursors (we can probably declare this out-of-scope but it's worth mentioning):

https://asciinema.org/a/HNbCisuhK554vUKP