Add a separate 'Description' property.

Currently, the already existing 'Comment' property is used to set the task subject and description. By introducing a Description property as well, we want to be able to set a separate task description if required. If the 'Description' property isn't set, we will still use the 'Comment' property (for backwards compatibility).

This involves changes in:

• human task handler to take the new property into account (mauricio)
• add 'Description' property by default in web designer (tiho)
• add 'Description' property by default in eclipse modeler (bob)