[ui.behavior] Fix transition terminal resolution in layouter

Make resolution of transition terminal nodes consistent. Fixes an infinite loop case, where a source state has both a self transition and also a transition that targets a substate transition point.

• Also added regression test in ui.behavior.tests

Issue #83 (closed)

Questions

• What about other uses of getTerminalNode() vs. getTerminalState() in DiagramEditingUtil? Is everything consistent there?
• Is it actually ok to do the layouting in the main thread?