GJS implements a basic signal system that allows monkey-patching
JS objects with signal methods resembling the GObject ones. However
it's clearly not a good idea to replace the actual GObject methods,
so use the proper GObject facilities when inheriting from GObject.
https://bugzilla.gnome.org/show_bug.cgi?id=778660
Just like keybindings and the message tray pointer barrier, gestures
don't always make sense - for instance, swiping up the screen shield
should not trigger the message tray just as the SelectArea action around
the left edge should not open the overview.
To avoid this, restrict gestures based on the current keybinding mode.
https://bugzilla.gnome.org/show_bug.cgi?id=740237
