Wayland: Support xdg-shell protocol

The wl_shell interface is deprecated in Wayland. A number of issues are caused by this:

• ClutterStageWayland doesn't work without a wl_shell_surface
• ClutterBackendWayland looks for the wl_shell interface even if the application doesn't use ClutterStage
• As a result of the above, calling clutter_init() with the wayland backend causes an infinite loop waiting for wl_shell if it doesn't exist

This PR fixes those issues by adding support for the replacement xdg_shell protocol. The changes have been tested to work in these compositors:

• gnome-shell
• kwin_wayland
• sway
• wayfire
• weston

Additionally, this has been tested to work with both the autotools and meson builds.