First Implementation

First working system; currently just add image supported
UndoRemoveLayer

Start to implement UndoRemoveLayer;
not finished yet
UndoRemoveLayer + BugFix

completion of UndoRemoveLayer;
some minor bugfix
UndoMoveXYLayer

Implemented UndoMoveXYLayer
UndoMoveZLayer

Added UndoMoveZLayer;
Fixed Bug: It was possible that a non changed Position created a
UndoMoveXYLayer
UndoGroupLayers & UndoUngroupLayers

Integration of UndoGroupLayers & UndoUngroupLayers
Minor Changes LayerTreeModel

Removed some unused functions