wayland: Refactor DnD target functions into MetaWaylandDragDestFuncs

This will be useful in order to interact with drag dest surfaces in
its windowing-specific ways, although everything defaults to the
wayland vfuncs.

https://bugzilla.gnome.org/show_bug.cgi?id=738312

src/wayland/meta-wayland-types.h

@@ -30,6 +30,8 @@ typedef struct _MetaWaylandPopupGrab MetaWaylandPopupGrab;
 typedef struct _MetaWaylandPopup MetaWaylandPopup;
 typedef struct _MetaWaylandKeyboard MetaWaylandKeyboard;
 typedef struct _MetaWaylandTouch MetaWaylandTouch;
+typedef struct _MetaWaylandDragDestFuncs MetaWaylandDragDestFuncs;
+typedef struct _MetaWaylandDataOffer MetaWaylandDataOffer;
 typedef struct _MetaWaylandDataSource MetaWaylandDataSource;
 typedef struct _MetaWaylandDataDevice MetaWaylandDataDevice;