This GdkEventController is a helper object to handle pad events,
it allows setting a mapping to action names, to be triggered in
the given action group.
In order to help on places where advanced mapping/configurability
of pad features is not desirable, this controller also allows
passing a NULL pad device, meaning it will listen on all pads,
and/or passing -1 on mode/index, so an action applies to all
modes/features (eg. strips/rings).
https://bugzilla.gnome.org/show_bug.cgi?id=770026
