Popover: Add get/set_transitions_enabled() and the property.
Popover: Add get/set_transitions_enabled() and the property.

johne53/MB3Gtkmm,johne53/MB3Gtkmm,GNOME/gtkmm,GNOME/gtkmm,johne53/MB3Gtkmm,GNOME/gtkmm,GNOME/gtkmm