Improve eventloop control stability. Add eventloop and multithread tests

[IanButterworth]

Changes:

• Removes the async eventloop stop. It was introducing too much instability.
  It was added to work around the MacOS bug where the window doesn't finish closing, which appears to be an upstream Gtk issue. https://gitlab.gnome.org/GNOME/gtk/-/issues/4648#note_1359210 . Perhaps we could leave this open for a few days and see if that gets fixed
• Makes pause_eventloop wait for the eventloop to be paused
• Fixes some bugs around stability
• Add tests covering eventloop control and explicitly tests the multithreading issue, when guarded
• Soft-deprecates Gtk.gtk_quit() -> Gtk.gtk_main_quit() as it matches the underlying function and gtk_main() better

[codecov]

Codecov Report

Merging #615 (6735b15) into master (ca39439) will increase coverage by 0.64%.
The diff coverage is 91.42%.

❗ Current head 6735b15 differs from pull request most recent head f7e0109. Consider uploading reports for the commit f7e0109 to get more accurate results

@@            Coverage Diff             @@
##           master     #615      +/-   ##
==========================================
+ Coverage   57.95%   58.59%   +0.64%     
==========================================
  Files          32       32              
  Lines        2721     2652      -69     
==========================================
- Hits         1577     1554      -23     
+ Misses       1144     1098      -46     

Impacted Files	Coverage Δ
src/GLib/signals.jl	76.95% <ø> (-1.42%)	⬇️
src/displays.jl	52.71% <ø> (+19.56%)	⬆️
src/events.jl	45.91% <0.00%> (-1.03%)	⬇️
src/Gtk.jl	86.11% <92.85%> (-5.96%)	⬇️
src/GLib/GLib.jl	61.53% <100.00%> (-15.74%)	⬇️
src/base.jl	79.74% <100.00%> (ø)
src/builder.jl	85.71% <100.00%> (+85.71%)	⬆️
src/gtktypes.jl	33.33% <0.00%> (-40.01%)	⬇️
src/GLib/gtype.jl	63.13% <0.00%> (-12.90%)	⬇️
... and 10 more

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[IanButterworth]

Tests are passing but I'm still exploring this PR.

It would be nice to remove the async eventloop stop, but I'm still getting windows staying open and freezing on macos.

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I'm exploring whether the macos thing is an upstream bug https://gitlab.gnome.org/GNOME/gtk/-/issues/4648

Update: it appears to be https://gitlab.gnome.org/GNOME/gtk/-/issues/4648#note_1358579