QmlDesigner.DesignDocument: getting rid of the magical StackedWidget

This StackedWidget is a leftover from the stand alone version.
In the context of Qt Creator this makes no sense anymore.

Change-Id: I6660dd4b43bdc12fa3329d333158fa759bee4d88
Reviewed-by: Marco Bubke <marco.bubke@digia.com>
QmlDesigner.DesignDocument: code cleanup

Change-Id: Ib71117cb5726662b4212f99a7c4136005273ae66
Reviewed-by: Marco Bubke <marco.bubke@digia.com>
QmlDesigner.CrumbleBar: code cleanup

Change-Id: I20e2002c7b2ccc25d8120222fa9b52e59db498ee
Reviewed-by: Marco Bubke <marco.bubke@digia.com>