unset WM_STATE for x11 clients upon unmapping them
icccm provides conflicting spec info regarding how this property should be set.
according to 220.127.116.11:
When the window is withdrawn, the window manager will either change the state
field's value to WithdrawnState or it will remove the WM_STATE property entirely.
however, 4.1.4 states:
Only the client can effect a transition into or out of the Withdrawn state
so to be on the safe(r) side, deleting the property seems to be the best choice here