Restructured the way shapes are stored inside the document. All shapes are now hold in a flat list. The hierarchical storage is provided by the layers. Each shape has a parent shape, which is either a layer or a group being a child of a layer. This was neccessary to provide a nice and clean hierarchical view of the document with the layer docker, which works really nice now. The layer docker can now be used to select and deselect shapes.

Additionally i fixed the ungrouping of groups by also deleting the group shape after ungrouping.