hamster: use gi.require_version for every gi module

Created by: mwilck

This should silence all "PyGIWarning: XXX was imported without specifying a version first." log messages.

Users have complained about these warnings repeatedly. This patch bundles all gi version requirements in one place, lib/gi_versions.py, and just imports that were necessary.

FTR: I understand that you're not putting maintenance efforts in this code base any more. However I wanted to share this with you anyway. I've taken ownership of hamster-time-tracker for openSUSE. In this role, I need and want to share all fixes I make for the upstream package with upstream. I'll checkout the new code base at some later time.