compositor: keep track of the weston_layer a weston_view is in

This introduces a new struct, weston_layer_entry, which is now used
in place of wl_list to keep the link for the layer list in weston_view
and the head of the list in weston_layer.
weston_layer_entry also has a weston_layer*, which points to the layer
the view is in or, in the case the entry it's the head of the list, to
the layer itself.
diff --git a/tests/weston-test.c b/tests/weston-test.c
index 35ccaa4..ca2f219 100644
--- a/tests/weston-test.c
+++ b/tests/weston-test.c
@@ -93,9 +93,9 @@
 	struct weston_test_surface *test_surface = surface->configure_private;
 	struct weston_test *test = test_surface->test;
 
-	if (wl_list_empty(&test_surface->view->layer_link))
-		wl_list_insert(&test->layer.view_list,
-			       &test_surface->view->layer_link);
+	if (wl_list_empty(&test_surface->view->layer_link.link))
+		weston_layer_entry_insert(&test->layer.view_list,
+					  &test_surface->view->layer_link);
 
 	weston_view_set_position(test_surface->view,
 				 test_surface->x, test_surface->y);