Configure menu buttons to show traditional submenus without sliding

[cameronwhite]

This makes it easier to browse through nested menus like the effects without annoying sliding and extra clicks to go back. The only minor annoyance is that the menus sometimes pop out to the left side, depending on available space if Pinta's window is near the right edge, but overall it feels nicer IMO

[pedropaulosuzuki]

Maybe we could reuse the CreateMenuButton function to, instead of creating each menu entry from MenuModel directly, use the Gtk.PopoverMenu.add_child method to create each entry (with icons, shortcuts and even tooltips, if we need to), and then use a Gtk.PopoverBin over a Gtk.Box when we have submenus. This allows greater control without making things too complex.