Move popovers to the container section

Makes more sense there.
Add an editor for GtkPopover
Initial GtkPopoverMenu support

Add GtkPopoverMenu with two virtual properties, one for the number
of menus, and one for the currently edited menu.
Make submenu switching work from the tree

Ensure that the submenu containing the currently selected widget is
visible.
Add an editor for GtkPopoverMenu
Handle submenu changes better

Deal with 'spontaneous' changes of the visible submenu. This will
happen once we add model buttons.
Initial GtkModelButton support
Allow creating themed icons