[GTK4 Prep] Replace menu items with menu model

[colinkiama]

I replaced as much as I possibly could.

The SourceView requires GTK4. We'd have access to Gtk.TextView.extra_menu.

Also, the PopoverMenus used for the replaced Gtk.MenuItems currently don't show keyboard shortcuts. As far as I know, this should be fixed after moving over to GTK4.

Part of #1157.

[colinkiama]

Sure, I'll split this up into multiple pull requests.