Port to GTK4

[danirabbit]

Things to do to prepare:

• Use Hdy.Tabbar
• Make Granite.SourceList Internal
• Replace Gtk.MenuItem with GLib.MenuModel

[colinkiama]

Make Granite.SourceList Internal

So does this mean that Granite.SourceList should be moved into the elementary Code codebase?

[danirabbit]

@colinkiama yes since there is no SourceList in Granite 7, Code will need its own sidebar class

[colinkiama]

Things to do to prepare:

• Use Hdy.Tabbar
• Make Granite.SourceList Internal
• Replace Gtk.MenuItem with GLib.MenuModel

Does "Replace Gtk.MenuItem with GLib.MenuModel" include the SourceList widget?
It's quite a bit of effort for something that will be completely be rewritten in the GTK 4 port. We also have the issue of Gtk.PopoverMenu not using elementary styling in GTK 3 (unless I'm missing something):

[danirabbit]

@colinkiama you can use Glib.Menu with Gtk.Menu for Gtk3 and then the port from Gtk.Menu to Gtk.PopoverMenu in Gtk4 is much smaller than going from Gtk.MenuItem all the way to menu models at the same time. Basically just trying to break things down as much as possible into reviewable branches so we're not trying to merge a several thousand line diff that nobody can reasonably review