Bugfix: always reset state->initial to false, not only for different stacking order

Bugfix: always reset state->initial to false, not only for different stacking order

As initial may be set to true again after initializing the window, we need to
ensure that it is properly cleared. Otherwise, this leads to ghost windows when
unmapping (such as the Xpdf about dialog) due to i3 issuing MapWindow after an
Unmap happened but before i3 actually received/handled the UnmapNotify.