Stop using lablgtk

[gdt]

lablgtk seems to be not really maintained, and a number of serious bug reports remain open. Long term, we should stop using it, which means either not having a GUI, using some other bindings from ocaml, or having some kind of control API and a GUI written in some other language. Or something else I haven't thought of.

Followups on -hackers please. This is simply a placeholder issue that the current situation is unsound.

[gdt]

This comment (to be edited over time) is the list of lablgtk issues that have an impact on unison:

• cast pointer to long
• naked pointer cast to ocaml value
• nonstatic pointer cast to C

[gdt]

This comment (to be edited over time) collects pointers to workarounds for issues with lablgtk:

• https://github.com/bcpierce00/unison/pull/1076