GtkModelMenuItem: add a submenu action attribute

GtkModelMenuItem: add a submenu action attribute

Add support for a stateful action associated with a submenu.  The action
state is set to TRUE when the menu is shown and FALSE when it is
unshown.

This is useful to avoid unnecessary processing for menus that have
frequently-changing content.

A possible future feature is to add support for asynchronously filling
the initial state of the menu by waiting until the action actually emits
its state-change signal to TRUE before showing the menu.

A silly example has been added to Bloatpad to demonstrate the new
feature.

https://bugzilla.gnome.org/show_bug.cgi?id=682630