Diagram Editor: provide option to enable manuel resizing of root diagram pane

[till-f]

It should be possible to resize the diagram pane on the right and bottom corner using the mouse (drag). The applied size should become fixed (no future update / auto resize) and stored in the model.

[alexanderpann]

Part one was implemented a while ago. You can resize it in all directions. The fixed size is not stored in the model though.