popover: Remove signal handlers in dispose
popover: Remove signal handlers in dispose

If we don't, they may get triggered at an inopportune
time later, causing us to crash.

See
https://bugzilla.redhat.com/show_bug.cgi?id=1203499

https://bugzilla.gnome.org/show_bug.cgi?id=747539

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