FR: change request for ProcessStack: use gdb.Frame/Symbol instead of string parsing

ModuleState.ProcessStack() currently uses gdb commands and parses its results. This is a bit error prone (the format is adjusted sometimes) and slower than necessary because those are all string operations and handled via GDB's user frontend instead of the python backend we're running in.

Please adjust that to use gdb.Frame and gdb.Symbol as in CFrameWorker.Go(). This should also fix the "show from a wrong frame because of missing debug info" issue.

In any case: please split ProcessStack into two functions "get the gdb.Frame we're interested in" and "process that gdb.frame` as this then should allow to implement #35 later, too.