hamster/client: edit_activity: don't touch self.date on date changes

Created by: lynxis

The fact is recalculated already in on_start_date_change(). By calling the setter of self.date the fact would be change a second time. The start_date was then wrong by the delta. E.g. edit a complete fact (start and end must be set). Click on the start date and choose 2 days into the future, the command line shows now 4 days into the future.

Fixes: #667 (closed)