Changed a Link to have a jump-over or a jump-gap with another link if they do not share the same start points or the same end points, even if they connect with the same node.
Improved Diagram.add to check that the argument Part is not already a part of a different Diagram.
Fixed copy and paste not to copy partly or fully disconnected Links.
Fixed the updating of jump-overs or jump-gaps in other links after adding or removing links.
Fixed undo/redo changes of z-ordering of Panel elements.
Fixed Group z-ordering when adding groups or switching layers.
