wayland: Add 'MetaWaylandPopupSurface' bridge between popup and surface

Add a bridge between the MetaWaylandPopup object and the corresponding
popup surface role. This bridge replaces communicating dismissed and
unmapped popup events.

https://bugzilla.gnome.org/show_bug.cgi?id=763431

src/wayland/meta-wayland-types.h

@@ -28,6 +28,7 @@ typedef struct _MetaWaylandPointerGrab MetaWaylandPointerGrab;
 typedef struct _MetaWaylandPointerGrabInterface MetaWaylandPointerGrabInterface;
 typedef struct _MetaWaylandPopupGrab MetaWaylandPopupGrab;
 typedef struct _MetaWaylandPopup MetaWaylandPopup;
+typedef struct _MetaWaylandPopupSurface MetaWaylandPopupSurface;
 typedef struct _MetaWaylandKeyboard MetaWaylandKeyboard;
 typedef struct _MetaWaylandKeyboardGrab MetaWaylandKeyboardGrab;
 typedef struct _MetaWaylandKeyboardGrabInterface MetaWaylandKeyboardGrabInterface;