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-keyboard.h

@@ -54,7 +54,7 @@
 #define META_TYPE_WAYLAND_KEYBOARD (meta_wayland_keyboard_get_type ())
 G_DECLARE_FINAL_TYPE (MetaWaylandKeyboard, meta_wayland_keyboard,
                       META, WAYLAND_KEYBOARD,
-                      GObject);
+                      MetaWaylandInputDevice)
 
 struct _MetaWaylandKeyboardGrabInterface
 {
@@ -81,7 +81,7 @@ typedef struct
 
 struct _MetaWaylandKeyboard
 {
-  GObject parent;
+  MetaWaylandInputDevice parent;
 
   MetaWaylandSeat *seat;