Show and hide child windows recursively

Change-Id: I021e380f59ec8edbee0dbb9f3369827c8e4e514d
printWindowHierarchy(): Iterate over QCocoaWindow::m_childWindows

... instead of -[NSWindow childWindows].

Change-Id: I9721e9f5c002c74f751934373f5922e5063d66fb