menubutton: Add `active` property and getter/setter

GtkMenuButton currently does not provide a way to tell if it's open programmatically. The existing methods, popup() and popdown(), do not expose any state to callers. If someone wanted to know whether or not a menubutton was open, they needed the popover. Given that GtkMenuButton can manage the popovers itself, that's not always an option for app developers.

This commit adds the active property and associated methods, where gtk_menu_button_set_active () replaces both gtk_menu_popup () and gtk_menu_popdown (). This addition also mirrors changes in other places, Such as GtkWidget:visible vs show()/hide().