Add source tab for the activity diagram.

Copied from Gary's comment.

When displaying the execution path (currently only possible for active process instances), it just shows the graphical representation. Was thinking that it would be good if this window had two tabs, like a lot of Eclipse editors, that show the graphical view on one tab, and a table of the actual events in the other view. Possibly have a split window on the 'source' tab, that shows the table at the top, with a panel at the bottom to show the XML representation of the event selected in the table. This same tabbed window could then be used to display active and historical process instances.