Stunner - Improve usability for connector bendpoints

Actually some people from the field complains about the usability for handling the connector's bendpoints.

Two proposals for this:

• Set/increase the selection area for the bendpoint (red circle), this way will be easier to click and drag
• [From dmarrazzo] - In order to improve the connection handling, I think that it's better to add a split point with the simple drag action instead of the double click.
  Basically, the behaviour of the legacy editor.
• Also UX proposes to change the color for the bendpoints from red to some other one (typically the red color is used in Stunner to indicate some error)