wayland: Add common object for device classes

Add a new object class, MetaWaylandInputDevice, and make all device
classes (pointer, keyboard, touch) inherit it. In the future common
functionality may be placed there.

https://bugzilla.gnome.org/show_bug.cgi?id=771305

src/wayland/meta-wayland-pointer.h

@@ -34,7 +34,8 @@
 
 #define META_TYPE_WAYLAND_POINTER (meta_wayland_pointer_get_type ())
 G_DECLARE_FINAL_TYPE (MetaWaylandPointer, meta_wayland_pointer,
-                      META, WAYLAND_POINTER, GObject);
+                      META, WAYLAND_POINTER,
+                      MetaWaylandInputDevice)
 
 struct _MetaWaylandPointerGrabInterface
 {
@@ -62,7 +63,7 @@ struct _MetaWaylandPointerClient
 
 struct _MetaWaylandPointer
 {
-  GObject parent;
+  MetaWaylandInputDevice parent;
 
   MetaWaylandSeat *seat;