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

Created by: ederag

#490 seemed to fix the shell-extension (https://github.com/projecthamster/hamster-shell-extension/pull/316#issuecomment-564145295), but that overrides the parsed fact start_time with hamster_now as a default.

Here is the 2012 state: https://github.com/projecthamster/hamster/blob/80110affd6f0c3a91d739c09a78dffe7c06976a1/src/hamster-service#L117-L121 https://github.com/projecthamster/hamster/blob/80110affd6f0c3a91d739c09a78dffe7c06976a1/src/hamster/lib/stuff.py#L322-L323 so start and end times were taken from the parse string, and overridden only if the separately given start_time and end_time were non-zero.

This prevents overriding the end_time with None for instance.

Proposition

Use 0 to mean "do not mess around with the parsed fact" (just ignore this argument) Use -1 as a special value to mean "replace with None"

This way the full power of the new parser (including datetimes and relative times parsing) can be exploited in the shell-extension as well.

For instance one might try this branch: https://github.com/ederag/hamster/tree/AddFact-parse-times