X10DT should provide an Call Graph view

Like the JDT's view of the same name, this would permit the user to start from a given method (not necessarily an external entry point), and browse the call graph in both directions (callers and callees).

The JDT uses a tree widget for this purpose, and thus has the problem that one can repeatedly traverse cyclic edge sets (with no easily visible indication). In theory, we could do something else, say, a proper Zest-based graph viewer. Not sure that's worth it, though.

In the near/mid term, users can probably make do with the "Search for References" mechanism.

We definitely have to have one or the other (Call Graph or Search for References).

Both options will require some form of search index, for efficiency's sake (especially given the speed of the compiler front end).