xwm: don't do anything except mapping on MapRequest
Instead, move the wlr_xwayland_surface_set_withdrawn() and wlr_xwayland_surface_restack() calls to the MapNotify handler with an override_redirect check, as they are done too early. This mirrors the logic in the UnmapNotify handler and fixes a bug where wlr_xwayland_surface_restack() would be called on an o-r window after the following sequence of requests:
- CreateWindow with override_redirect=True
- ChangeWindowAttributes with override_redirect=False
- MapWindow
Fixes: https://gitlab.freedesktop.org/wlroots/wlroots/-/issues/3770