GtkMenu: Properly detach menu when the window is detached
GtkMenu: Properly detach menu when the window is detached

Without properly cleaning up GtkMenu private attach state
(GtkMenuAttachData) when the attached widget is freed, we would end up
with an invalid pointer to a freed widget. Trying to detach from that
widget would cause a segmentation fault.

https://bugzilla.gnome.org/show_bug.cgi?id=752761

jadahl/gtk,davidgumberg/gtk,grubersjoe/adwaita,davidgumberg/gtk,jigpu/gtk,Adamovskiy/gtk,alexlarsson/gtk,Adamovskiy/gtk,chergert/gtk,alexlarsson/gtk,chergert/gtk,chergert/gtk,Adamovskiy/gtk,alexlarsson/gtk,Adamovskiy/gtk,Adamovskiy/gtk,alexlarsson/gtk,grubersjoe/adwaita,chergert/gtk,jadahl/gtk,davidgumberg/gtk,davidgumberg/gtk,alexlarsson/gtk,jigpu/gtk,grubersjoe/adwaita,Adamovskiy/gtk,jadahl/gtk,chergert/gtk,jigpu/gtk,jigpu/gtk,grubersjoe/adwaita,grubersjoe/adwaita,jigpu/gtk,alexlarsson/gtk,chergert/gtk,grubersjoe/adwaita,jadahl/gtk,jadahl/gtk,grubersjoe/adwaita,davidgumberg/gtk,davidgumberg/gtk,chergert/gtk,Adamovskiy/gtk,alexlarsson/gtk,jadahl/gtk,alexlarsson/gtk,davidgumberg/gtk,jigpu/gtk,jadahl/gtk,chergert/gtk,jadahl/gtk,jigpu/gtk,grubersjoe/adwaita,jigpu/gtk,davidgumberg/gtk,Adamovskiy/gtk