cursor: allow re-focusing xwayland-unmanaged surfaces

cursor: allow re-focusing xwayland-unmanaged surfaces

We already allow some xwayland-unmanaged surfaces to take focus on map,
if indicated by wlr_xwayland_or_surface_wants_focus(). But once these
surfaces lose focus, they never regain it again.

Add desktop_focus_view_or_surface() and call it in the appropriate
places to allow these views to regain focus in the usual ways (e.g.
clicking on them or focus-follows-mouse).