Missing time overlap check

[gstuer]

At the moment overlapping times of entries associated to one certain date are not going to lead to an exception. This should be changed to avoid wrong time and pause calculations.

Example: EntryOne on date 28.11.2019 with starting time 11:00 and end time 16:00 and EntryTwo on date 28.11.2019 with starting time 13:00 and end time 14:00.
=> Trivially invalid, but no check for this overlap implemented yet.