Add api to allow explicitly setting a GtkPopover instead of
a GtkMenu as the popup of a GtkMenuButton. Also, add api to
instruct the menu button to construct a popover when given
a menu model.

We set the style class "menu-button" on the button only when
it pops up a menu, to allow different treatment for the active
state of the button in the two cases.

https://bugzilla.gnome.org/show_bug.cgi?id=723878