Description of problem:
It is not possible to add a boundary event to an ad-hoc subprocess.

Version-Release number of selected component (if applicable):
ER6

Steps to Reproduce:
1. Create a new business process and add an Ad-Hoc subprocess to the canvas.
2. Try to attach a new catching event to the subprocess.

Actual results:
It is only possible to add it inside the subprocess as an intermediate event.

Expected results:
There should be a green border around the subprocess signalizing you can create a boundary event. (This is how it behaves when you want to add a boudary event to an embedded subprocess.)