Correctly lay out boundary to activity loops

[barmac]

Summary of changes:

• we no longer snap to target center during connection
• we snap inside of the target element
• we snap Boundary Event loops in the way to make sure the connection end is not too close
• layouter checks the area where cursor is placed to determine loop layout hints

Required by camunda/camunda-modeler#903

See screenshots in the linked issue for the context.

[nikku]

I like the behavior for tasks (with a specified size).

However, it does not seem to be the right behavior for sub-processes:

[nikku]

We discussed this and agreed on looking into the connection behavior with sub-processes, too.

[barmac]

It's not WIP anymore. @nikku can you please have a look at this once more?

Summary of changes:

• we no longer snap to target center during connection
• we snap inside of the target element
• we snap Boundary Event loops in the way to make sure the connection end is not too close
• layouter checks the area where cursor is placed to determine loop layout hints