view/xwayland: avoid focusing views that don't want focus

XWayland views can self-declare that they don't want keyboard focus via
the ICCCM WM_HINTS property. Most of the logic is already in place to
avoid giving focus to such views (e.g. taskbars).

Add a couple of missing pieces to make this work:

- Hook up view_isfocusable() to look at WM_HINTS for XWayland views
- Adjust desktop_focus_topmost_mapped_view() to skip unfocusable views

src/xdg.c

@@ -554,7 +554,7 @@ xdg_toplevel_view_unmap(struct view *view, bool client_request)
 		view->mapped = false;
 		wlr_scene_node_set_enabled(&view->scene_tree->node, false);
 		wl_list_remove(&view->commit.link);
-		desktop_focus_topmost_mapped_view(view->server);
+		desktop_focus_topmost_view(view->server);
 	}
 }