substitute mapping_state == Withdrawn by m_managed

as the flamewar pointed out, resetShowingDesktop invalidly keeps hidden windows
the reason is that there're several updateVisiblity calls (notably one from the compositor)
which  break the showingDesktop state as a side effect (before ::manage() does it's thing)

Since they also invalidate the Withdrawn mapping state, that will fail as isManaged() test
(it's also invalidly used by the compositor to set up the decoration, isManaged() used to be
true because of the updateVisibility() call before) since the result is never Withdrawn

CCBUG: 299655
REVIEW: 105303