Stunner - Fix issues with improved usability of bend points

Fix issues with improved usability of bend points:

• When there are two parts of one sequence flow too close to each other, when trying to create one bend-point, two bend-points are created. There should be created only one.

• If there is too short sequence flow (e.g. when you create any node via quick menu), it is not possible to create and move with bendpoint.

• It is possible to move with bend-point outside of the canvas, and after doing so, it is not possible to move with it. Also Undo action does not help with it.