timetracker

Version: v0.10.0
To reproduce, go to settings and add more than categories at the same time below any category.
When you assign the name for one of the new, the other get the same name.
If you go to the second and try to change the name, the first one gets the second name.
I hope this helps.
Thanks.

[aw-qt_2021-03-26T00-09-49.log](https://github.com/ActivityWatch/activitywatch/files/621747

Is your feature request related to a problem? Please describe.
Bookings with same start and stop time are making no sense, but are now possible

Describe the solution you'd like
Prevent these bookings.
If there is another sense behind this behavior make it optional via settings.

There should be Table Driven Tests for core/formatter.go. They should be structured similar to the tests in core/timetrace_test.go.

Browser is already using localization for dates and times, using toLocale on all dates. We just need to localize all text.