bypass (0) and override to None (-1) start/end in DBus AddFact

Created by: ederag

This hopefully fixes #491 (closed). It should be backward compatible with existing workflows, while allowing to enter start and end times as well, for people interested.

This is the last commits of the experimental branch described in https://github.com/projecthamster/hamster/issues/491#issuecomment-564730079, rebased on master.