date/time entry consistency

Created by: ederag

1. When date and time can be given explicitly together, e.g. 2019-09-01 02:00, they should be local time (the same as on the desktop wall clock), as in hamster-cli. This is seems to make more sense when entering both start and end datetimes: 2019-09-01 04:00 - 2019-09-01 07:00

2. When a day is selected separately, as with the gui timeline arrows in activity_entry, then it means "hamster day" (that by default starts at 05:00). This way the activity will appear in exactly this selected day when displayed by the overview, hamster list, hamster search, ...

3. When only time is entered, the default date should be hamster_today. Indeed, entering 23.50 half an hour later refers most probably to the current hamster day. (using today would be very inconvenient here, think of entering 23.50 - 00.20...).

4. Facts overlapping the hamster day start time (05:00 by default) are tricky. e.g. entering 04:00 - 07:00 on 2019-09-01 at 07:30 would result in a 2019-09-02 04:00 - 2019-09-01 07:00, hence with a negative duration. This has been solved for the gui in PR #437 by incrementing the end date (more secure than decrementing the start date) in case of negative duration. A warning and a means to check and correct entry was given. For command line this is a bit trickier. Just error on negative durations and require user to give full datetime information ?