Simplify 'late' switching

[ramack]

It is quite common to forget to switch the activity in time and when you touch your phone the next time it was not even possible to fix it before 1.2.0 but editing the history is still tedious.

We should add a way to change the activitiy "late" by selecting how long ago, or at what time we actually changed it. Maybe in the snackbar we can add another action beside the "UNDO" to select the time of switching.

[ramack]

Maybe as #156

[ramack]

having #156 solved is acceptable workaround I'd say, but still the option in the snackbar to open from there a start-time-picker would be good in the "middle" future.