[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.

[danirabbit]

Haven't tested this yet or looked into it too much, but just made a note about code style and detailed action names.

Since there's issues in Gtk3 with PopoverMenu, you can use Gtk.Menu with a model https://valadoc.org/gtk+-3.0/Gtk.Menu.Menu.from_model.html and then it's a pretty easy drop-in replacement during the GTK 4 port to change that

Since this is quite a huge branch I wonder if it might be better to do this in smaller steps, like just a branch to switch to using GLib.Actions I imagine is already a huge review in itself. It might help to get your work merged in faster if branches are more incremental

[danirabbit]

Code style for wrapping should be like:

Suggested change

	var open_in_terminal_pane_item = new GLib.MenuItem (_("Open in Terminal Pane"),
	MainWindow.ACTION_PREFIX
	+ MainWindow.ACTION_OPEN_IN_TERMINAL
	+ "::"
	+ file.path);
	var open_in_terminal_pane_item = new GLib.MenuItem (
	_("Open in Terminal Pane"),
	MainWindow.ACTION_PREFIX
	+ MainWindow.ACTION_OPEN_IN_TERMINAL
	+ "::"
	+ file.path
	);

There's actually a function for creating detailed action names as well: https://valadoc.org/gio-2.0/GLib.Action.print_detailed_name.html