Squish: Test "Go to slot" in Widget Designer

Change-Id: Idc4517f86035aa5faa1265f275b0ed5f97526d2f
Reviewed-by: Christian Stenger <christian.stenger@digia.com>
Fix issues with having same document open in multiple splits

Sometimes one split would not show the document even though it shows in
the combo box.

Task-number: QTCREATORBUG-9801
Task-number: QTCREATORBUG-9798
Change-Id: Iea79c6f3deb4f1570685794c6eb762f603a36b5f
Reviewed-by: Eike Ziller <eike.ziller@digia.com>