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-types.h

@@ -23,6 +23,7 @@
 typedef struct _MetaWaylandCompositor MetaWaylandCompositor;
 
 typedef struct _MetaWaylandSeat MetaWaylandSeat;
+typedef struct _MetaWaylandInputDevice MetaWaylandInputDevice;
 typedef struct _MetaWaylandPointer MetaWaylandPointer;
 typedef struct _MetaWaylandPointerGrab MetaWaylandPointerGrab;
 typedef struct _MetaWaylandPointerGrabInterface MetaWaylandPointerGrabInterface;