Use wl_global_remove in libweston

Instead of having core, plugins, whatever call wl_global_create() and wl_global_destroy() directly, offer an API that wraps a struct wl_global and uses wl_global_remove() from wayland!28 (merged) under the hood to prevent clients racing with the removal. Naturally this requires tracking the new structs in struct weston_compositor and the associated timeout to finally destroy them after removal.

Use this new API for all wl_globals everywhere in the Weston repository.