Implementation of ACT-895

Finished implementation of ACT-895.

Give it a try and tell me what you think. The following things are possible:

* When a Call Activity is selected, in the "Main Config" property sheet, the 'Called Element' can be specified as a process ID of the other process to call. In case this process is found in any open Activiti project in any found diagram, the most-right button with the arrow to the right is enabled. It allows to open the diagram that references this process. In case multiple processes with the given ID exist in open projects, a dialog appears to choose the correct one.

* When a valid, unique process ID is given in the properties of a Call Activity, the context menu of the Call Activity will contain a new menu item "Open Process '<process-id>' in new diagram" (just realized it would be better to call this 'Open Process '<process-id>' in new Diagram'. I've decided to not implement the dialog here that allows to select in case the process ID is not unique. If you think this is a good idea, just drop me a line, we can change this easily.

Currently, I needed to copy the method to open a diagram by it's data file resource as we do not have a central static version of this. This might be something we can put in the utility project. Any idea where?

To accomplish this issue, I need to refactor quite some parts of the matching strategy of the diagrams, cause this was not really working (it matched the first diagram found, not the one that really fit). I did so by introducing a new ActivitiDiagramEditorInput.java in the eclipse project that references both, the diagram file resource (the bpmn2d) and the data file reference (the bpmn). Now it is very easy to match two diagrams simply by the editor input. 

Maybe this is handy on some other places too cause I've seen there was quite some energy spent in converting the two resource files into each other manually.