Create simplified UML Activity Diagram type and palette

[planger]

The UML Activity diagram shall include the UML concepts listed below. This includes adapting the palette.

• [ ] Activity
• [ ] Initial Node
• [ ] Final Node
• [ ] Activity final
• [ ] Decision node
• [ ] Merge node
• [ ] Join node
• [ ] Fork node
• [ ] Opaque Action
• [ ] Comment
• [ ] Control Flow
• [ ] Link
• [ ] Action
• [ ] Call BehaviorAction
• [ ] CallOperation Action
• [ ] AcceptEventAction
• [ ] Pin
• [ ] InputPin
• [ ] OutputPin

[cdamus]

Per discussion in #10, we need to add ObjectFlow to the Acitivty Diagram:

• palette
• assistant

[planger]

Closed via ccda4cf76348a429aaac28aad7a6710e0c9fd251