Fix the crash in Konsole with Qt 4.5.

The problem is that ViewContainer is a QObject and is not aware that one of its QWidget member can die because the ViewContainer is not part of the "GUI" hierarchy. So when you delete the main window it will first delete all QWidgets and then all QObjects, so in our case all GUI objects and then ViewContainer. But since ViewContainer react to destroyed signal of some GUI object and use it that explain the crash. Having QPointer is the easy to way to be aware if one of the member die because the GUI has been deleted.