Migrate from GTK 3 to GTK 4

[vanillajonathan]

Here is the GTK migration guide:
https://docs.gtk.org/gtk4/migrating-3to4.html

Commit ac2c123 was the first step!

[mvo5]

Hey, yeah, I added ac2c123 for this. I would love to port it eventually but help would be welcome.

[albert-tomanek]

@mvo5 I can help with this. Can you think of any parts of the code right now that are incompatible with Gtk4?

[mvo5]

@albert-tomanek Thanks, help would be much appreciated. I don't know if there is anything incompatible in the code, it is now build with -DGDK_DISABLE_DEPRECATED and -DGTK_DISABLE_DEPRECATED and that works. Historically the gtk/rgpkgtreeview.cc has seen tricky because it is a custom listview/treeview (for performance reasons). Most of the other pieces are relatively standard gtk. But keep in mind this went all the way from gtk1->gtk2->gtk3 so the code has a lot of history (and unfortunately it shows in some areas!). But I hope this does not scare you away, I would love to get help and start a gtk4 branch!

[vanillajonathan]

@albert-tomanek You're giving it a go?