gdk: Add pad event structs, enum values, and event mask bit

gdk: Add pad event structs, enum values, and event mask bit

GDK_PAD_BUTTON*,RING and STRIP will be emitted respectively when
pad buttons, rings or strips are interacted with. Each of those
pad components belong to a group (a pad can contain several of
those), which may be in a given mode. All this information is
contained in the event.

GDK_PAD_GROUP_MODE is emitted when a group in the pad switches
mode, which will generally result in a different set of actions
being triggered from the same buttons/rings/strips in the group.

https://bugzilla.gnome.org/show_bug.cgi?id=770026