unset WM_STATE for x11 clients upon unmapping them

icccm provides conflicting spec info regarding how this property should be set.
according to 4.1.3.1:

When the window is withdrawn, the window manager will either change the state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Only the client can effect a transition into or out of the Withdrawn state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

ref 41daddeacc

fix T3011

src/bin/e_hints.c

@@ -1289,6 +1289,7 @@ e_hints_window_hidden_set(E_Client *ec)
         ec->netwm.state.hidden = 0;
         EC_CHANGED(ec);
      }
+   ecore_x_window_prop_property_del(e_client_util_win_get(ec), ECORE_X_ATOM_WM_STATE);
    _e_hints_process_wakeup(ec);
 #endif
 }