[Feature] Add a shift system

[carnager]

If one wants to track worktime the reason often is to have an easy way of seeing days that differ from usual work shift.

I propose a way to define shift plans inside of trackworktime. Once that is in place trackworktime could easily recognize if the user worked different to usual shifts and calculate the offset.
Another possible future feature could be to automatically round the tracked worktime to match the shift for that day (with a configurable margin)

Such a system should have a few key elements:

• define work schedules for each day of the work-week
• a system to rotate shiftplans (maybe even allow plans to be associated with even/uneven week numbers)
• maybe more?