python3 port + usability fixes

Created by: ederag

hamster-time-tracker was dropped from openSUSE Leap15 https://www.reddit.com/r/openSUSE/comments/7zrj03/whats_the_best_time_tracker_on_tumbleweed/ The new development seemed to go well, but still claimed as alpha. The fork by gsobczyk had too many unresolved dependencies on Leap15. The master branch worked out of the box, without python-gconf, and with the same hamster.db database.

Several usability fixes were needed though. Now it seems more intuitive and consistent. There is a python2 branch, but I'm now exclusively on the python3 branch. There seems to be little chance to merge it back here, so comments, issues and small PR are welcome in this fork. It will hopefully last only a few years, until the new hamster-* is stable.