Allow buttons for CustomServiceTask properties that execute a custom class for string input

Investigate as described in this forum thread:

http://forums.activiti.org/en/viewtopic.php?f=8&t=2305&p=9847#p9847

"ACT-414 covers my request in a more generalized manner and I can wait for that feature. If a button control that invokes a user implemented class and returns a string could be implemented in a nearer timeframe that would be greatly appreciated. In the meantime I think I can just create an Eclipse plug-in for our custom configuration UI and have the developer paste the results into a text control."