Upgrade to GTK3

[griffi-gh]

replaces gtk2 with gtk3, adds various minor ui improvements

status: ready to merge! 🥳

most (if not all) stuff works, all deprecated functions except gtk_popup_menu are removed

TODOs

• [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)
  • [x] fix dark theme
  • 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
• [X] make buttons in Display settings perfectly square
• [X] clean up Settings-related code (single api for adding settings rows, use gtkgrid)
  • [X] right-align text
  • [X] replace help icon
  • [X] Tabs
  • [X] Graphics
  • [X] Audio
  • [X] Controls
  • [X] Interface
  • [X] do the same to level properties
  • [X] Tabs
    • [X] Info tab
    • [X] World tab
    • [X] Physics tab
    • [X] Gameplay tab
• [x] fix gtk3 deprecation warnings (this is the hardest part)
  • [X] remove gtk_threads_start and gdk_threads_enter/gdk_threads_leave
  • [X] replace gtk_settings_set_*_property with g_* functions
  • [X] remove references to GtkMisc
  • [X] replace Color with RGBA
  • [X] replace ColorsSelection with ColorChooser
  • [X] replace GtkH* GtkV*
  • [X] remove GTK_STOCK_*, gtk_image_from_stock, ...
  • [X] replace gtk_flush() with gdk_display_flush(...)
  • [X] replace state with state flags
  • [X] remove deprecated gtk_tree_view_set_rules_hint
  • [X] remove gtk_scrolled_window_add_with_viewport
  • [x] replace gdk_keyboard_grab with gdk_seat_grab
  • [x] remove gtk_dialog_get_action_area, as access to action area is deprecated and discouraged (pack widgets into content area instead)
  • [X] remove "pressed" event
  • [x] replace GtkTable with GtkGrid (used all over the codebase 😰)
  • [X] Settings
  • [X] Level properties
  • [X] Factory/robot factory
    • ❓ make it display "None" instead of "-1" in the index field
    • [ ] 🐛 MINOR BUG: make it not freeze when sorting by id, in reverse order (happens on gtk2 too)
  • [X] Treasure chest
    • [X] use buttonbox to make buttons connected
    • ❓ Remove selected - maybe add a [Remove] button next to individual entries instead
  • [X] "Set Frequency Range" dialog
  • [X] Robot settings
    • [x] make the right pane scrollable
    • [X] fix confusing radio buttons
  • [X] Cursor field
  • [X] Shape Extruder
  • [X] Polygon
  • [X] Synthesizer
  • [X] Timer
  • [X] Sequencer
    • [X] update step count in realtime
  • [X] replace all background overrides with css/alternative widgets
  • [X] Replace color picker Button in level properties with ColorButton
    • [x] hide the color picker if the current background can't be colored
    • [X] fix ColorButton spawning dialog behind. (215045015cf213e61f2951a27c638ff238f7e85f)
  • [X] misc. other deprecated calls
  • figure out what to do with popup_menu
  • [x] one of them can be converted to the new api
  • others will probably stay for now? the new api requires a parent window which we just don't have
• [X] replace gdkkeysyms-compat with gdkkeysyms (7faa553e820b7b61abd7721e81eb0f22898f0b17)
• [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 (if found)
• [x] replace button-release-event with click
• [x] remove gtk_window_set_keep_above as it causes a lot of issues
• [x] add missing delete level button in right click menu
• port the custom theme work to gtk3 css and re-enable it
  • [X] instead, enable prefer-dark-theme
• 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)

nice-to-have:

• Add a compile-time option/define that makes the game use GtkGlArea in a GtkWindow instead of sdl2, solves a lot of issues, including the popup menu ones. (maybe in a separate pr)

next:

• [?] test everything (need help)
  • [ ] test nix and fedora builds