Start of implementation for ACT-895

* Rewrote diagram editor input handling in ActivitiDiagramEditor by removing coupling to data- and diagram file.
* Introduced subclassed editor input ActivitiDiagramEditorInput that
references both data- and diagram file
* Made sure the newly subclassed version of the diagram editor input is
used when generating diagrams (either automatically via Eclipse IDE or
by wizard).
* Rewrote Matching Strategy so it correctly matches open editors, e.g.
by clicking the appropriate arrow in the preferences of a call activity. 
* Reactivated arrow in PropertyCallActivitiSection as a basic start.