fix focus of the OpenEditorsWindow

Commit 4b828173916c26b52bd2911a6ea28926e2a3a7a9 changed the parent of
the OpenEditorsWindow to 0, which is why it didn't get the keyboard
focus anymore.

Task-number: QTCREATORBUG-13196
Change-Id: I2fbf90bebfcab251e985fb80f5dfda16fd477d1c
Reviewed-by: Eike Ziller <eike.ziller@theqtcompany.com>