Editor: Automatically adjust size of open editors window.

Task-number: QTCREATORBUG-10618
Change-Id: If17fbb85596c9ea4d6a42eb99566b843812bd7fc
Reviewed-by: Knut Petter Svendsen <knutpett@pvv.org>
Reviewed-by: Eike Ziller <eike.ziller@digia.com>