re-opened from the new repo: https://github.com/Bithack/principia/pull/124

status: most (if not all, needs testing!) things work with tons of compile-time warnings.
windows theme is currently disabled.

todo:

[X] fix gtk2 deprecation warnings
[X] build with gtk3
- [X] windows (builds, works)
- [X] linux (builds, works)
[X] replace gtk-undo with gtksourceview-4
- [X] remove dependency on gtk-undo
- [X] integrate GtkSourceView
- [X] enable basic qol features (smart tabs, line numbers, syntax highlighting, etc)
- ~~enable basic autocomplete (just suggest some keywords and globals, and existing tokens)~~
- ~~it would've been cool to also add a simple syntax error checker (just compile on change and display the error)~~
- both of these are out of scope of this pr, and will likely come separately
[X] update build config
[X] update readme/documentation
[ ] WIP 🚧 fix gtk3 deprecation warnings (this is the hardest part)
[ ] WIP 🚧 replace background overrides with css
[X] fix critical errors on start (required to make the release mode work)
[X] fix object import menu, it was broken before but it straight up crashes on gtk3 due to OOB access.
[ ] fix minor widget size/layout issues
[ ] port the custom theme work to gtk3 css and re-enable it
- [ ] check if it works in debug mode on windows? (maybe gtk3 fixed the "fails spectacularly" thing?)
~~re-enable custom layout for robot config menu item dropdowns (?)~~ WONTFIX as you're not supposed to do that on wrapper types...
that works on gtk2, but fails on gtk3 (not supposed to be used anymore)

Bithack / principia