widget: Avoid critical warnings when disconnecting plain GtkEventControllers
widget: Avoid critical warnings when disconnecting plain GtkEventControllers

This signal is only set on GtkGesture objects, so check it's really there
before disconnecting.

grubersjoe/adwaita,grubersjoe/adwaita,grubersjoe/adwaita,grubersjoe/adwaita,grubersjoe/adwaita,grubersjoe/adwaita,grubersjoe/adwaita,grubersjoe/adwaita