Allow logging of new FicTrac data while playing back another log

[hubbardp]

Chad says that he wants to be able to play back a log file, and at the same time create a new log of the motion reported by FicTrac during the playback. The new motion would appear only in the new log file, and would not affect the display generated by playing back the old log file.

Code for this change is in the "log-during-playback" branch and this pull request:
#214