Stunner - it is possible to corrupt diagram and lose all made changes

If you try to connect Event sub-process with itself you will see an error (in attachment) and it is not possible to continue your work, all changes will be lost.

P.S. I guess we can disable quick menu entirely for Event sub-process (except delete and morph buttons) since it is not possible to append nodes to Event sub-processes.

Manual PR Acceptance test

• [Stunner] Try connect to itself
  • Start events
  • Intermediate events
  • End events
  • Tasks
  • Subprocesses
  • Gateways
• [Stunner] save and reopen
• [Stunner] moving existing connection to achieve self-reference
  • Not possible achieve self-reference, however user is not informed with error/warning