Detect QMainWindow-separator as passive interactor in Qt Designer.
Detect QMainWindow-separator as passive interactor in Qt Designer.

Ensure that dock widgets in Qt Creator can be resized.

Task-number: QTCREATORBUG-8284
Change-Id: If6aa55b2dde8fce53e1527db678f518b3912907b
Reviewed-by: Jarek Kobus <021d51876099fe64879cc5072886d1d0ff71e1ac@digia.com>

mer-qt/qttools,mer-qt/qttools,mer-qt/qttools,mer-qt/qttools,mer-qt/qttools,mer-qt/qttools