The override_redirect flag can change on configure notify and
on map notify. This adds an event to know when it changes.
This removes wlr_xwayland_surface_is_unmanaged which was wrongly
using the window type to decide whether the view should be
unmanaged.
A similar patch was proposed to Weston, but has never been
merged upstream [1].
[1]: https://patchwork.freedesktop.org/patch/211161/
153f37bdf5