wayland: Unmap popup windows when a popup chain is dismissed

When dismissing a popup grab, always unmap every popup window in the
chain, instead of relying on the surfaces and xdg_popups being
destroyed.

https://bugzilla.gnome.org/show_bug.cgi?id=744452

src/wayland/meta-wayland-surface.h

@@ -100,6 +100,12 @@ struct _MetaWaylandSurface
   MetaWaylandSerial acked_configure_serial;
   gboolean has_set_geometry;
 
+  /* xdg_popup */
+  struct {
+      MetaWaylandPopup *popup;
+      struct wl_listener destroy_listener;
+  } popup;
+
   /* wl_subsurface stuff. */
   struct {
     MetaWaylandSurface *parent;