wlr_xdg_toplevel: reparent on parent unmap
Created by: RedSoxFan
Fixes swaywm/sway#4421
From the xdg-shell specification:
If the parent is unmapped then its children are managed as
though the parent of the now-unmapped parent has become the
parent of this surface. If no parent exists for the now-unmapped
parent then the children are managed as though they have no
parent surface.