Fix bug #17508791: Sequence numbers increment incorrectly for videos when Synchronize RAW+JPEG enabled.

Fix bug in sequence letter generation.

Enable the disabling of new version checks in both the program logic and the user interface, which is useful for Linux distributions. To disable the check, Linux package maintainers should patch the file constants.py to change the line disable_version_check = False to disable_version_check = True. Of interest to @anarcat and others who package the program for a Linux distribution.

Include tornado as a dependency instead of relying on the deprecated mini-tornado found in pyzmq.