Debugger expressions

Some notes:
(1) This hasn't been rebased against the current master, but I don't think anyone has been working on the debugger, so there shouldn't be any conflicts.
(2) This includes an (out-dated) version of the UI change to the interpreter that is submitted in this PR: https://github.com/scala-ide/scala-ide/pull/489 (which I actually need to attend to!).
(3) Here's the gist to the REPL assistance: https://gist.github.com/jhaberstro/2a5f71093c6ac4ee754e