Remove xfw_(window|application)_get_id() and add xfw_window_get_xid()

@Tamaranch does this look ok to you? I feel like I might be missing something for the docs, as I always forget something there...

I think I'm going to hold off on adding xfw_window_wayland_get_foreign_toplevel_handle(), since presumably at some point we'll support both the wlr-foreign-toplevel-management as well as ext-foreign-toplevel-list (plus whatever other ext- protocol ends up being standardized to manage them).