MetaWaylandSurface: Allow passing parameters when assigning role

Allow passing parameters (only GObject parameters supported for now) so
that role assignment can affect the paremeters set during construction.

If a role was already assigned when assigning, the passed parameters
are set using g_object_set_valist().

https://bugzilla.gnome.org/show_bug.cgi?id=769936

src/wayland/meta-wayland-data-device.c

@@ -1174,7 +1174,8 @@ data_device_start_drag (struct wl_client *client,
 
   if (icon_resource &&
       !meta_wayland_surface_assign_role (icon_surface,
-                                         META_TYPE_WAYLAND_SURFACE_ROLE_DND))
+                                         META_TYPE_WAYLAND_SURFACE_ROLE_DND,
+                                         NULL))
     {
       wl_resource_post_error (resource, WL_DATA_DEVICE_ERROR_ROLE,
                               "wl_surface@%d already has a different role",