frame-filters (disabled), new command cbacktrace (#36)

... and minor improvements for internal frame handling, now cares for "outer" COBOL frames, too

supersedes !73 (closed), please see more details there. Mainly created to not diverge more and to finally fix #36 (closed) (the necessary filter is temporarily enabled, then disabled).

fixes #36 (closed), but is only registered for GDB 8.3+ (Python Frame-Filter API came with GDB 7.7), frame-filters are disabled by default, the planned "COBOL-FrameFilter" is disabled until #58 (closed) is solved, the version check can possibly be adjusted to 7.7 once !78 (merged) is integrated (!73 (closed) was actually tested against GDB 8.1, so I'll then drop to this version, also tested back then: raises no issue with GDB 7.6 (the command is just not available, similar to "good COBOL watch") and tested with Python2 and Python3 "mind the iterators").