Stunner - Docking issues

General review of the docking feature is required. Different issues found:

• Once saving and loading -> the dock shape does not appear on the UI (although present in the model)
• Once resizing a shape, which has some connections and a docker node, saving the diagram and re-opening it, results in wrong use of magnet/connections and wrong dock shape location.
• Test changing the parents for shapes with some docked elements
• Test undo/redo with dock operations
• Dragging Events from the palette should be allowed to be docked on tasks on the Canvas
• Review docking rules for BPMN elements
  • For example, BPMN only allows having docked nodes to just some certain task types, but actually Stunner allows it in all task types)
  • Anyway the idea is to check docking rules for all actually supported nodes and ensure they're correct