xwayland: associate/dissociate/map/unmap cleanups

- connect/disconnect map handlers in set_surface()
- call set_surface() at time of associate/dissociate

This separates the concepts of "associate" and "map" more clearly.

It's no longer necessary to listen for wlr_surface "destroy" event,
because dissociate is always received first.

Also, view->content_tree is now destroyed and set to NULL at unmap.
Previously, we relied on wlr_scene to destroy it automatically when
the surface was destroyed, but kept a potentially dangling pointer in
view->content_tree until next map. Similar change for unmanaged.

v2: comment updates

src/xwayland-unmanaged.c

@@ -68,7 +68,6 @@ handle_map(struct wl_listener *listener, void *data)
 		seat_focus_surface(&unmanaged->server->seat, xsurface->surface);
 	}
 
-	/* node will be destroyed automatically once surface is destroyed */
 	unmanaged->node = &wlr_scene_surface_create(
 			unmanaged->server->unmanaged_tree,
 			xsurface->surface)->buffer->node;
@@ -128,13 +127,15 @@ handle_unmap(struct wl_listener *listener, void *data)
 
 	wl_list_remove(&unmanaged->link);
 	wl_list_remove(&unmanaged->set_geometry.link);
-	wlr_scene_node_set_enabled(unmanaged->node, false);
 
 	/*
-	 * Mark the node as gone so a racing configure event
-	 * won't try to reposition the node while unmapped.
+	 * Destroy the scene node. It would get destroyed later when
+	 * the wlr_surface is destroyed, but if the unmanaged surface
+	 * gets converted to a managed surface, that may be a while.
 	 */
+	wlr_scene_node_destroy(unmanaged->node);
 	unmanaged->node = NULL;
+
 	cursor_update_focus(unmanaged->server);
 
 	if (seat->seat->keyboard_state.focused_surface == xsurface->surface) {