Split entries on new overlapping event

[akadaedalus]

Similar to issue #347 but still a different request.

I use "timewarrior" to track my work day. Sometimes I forget to clock in when I do a separate activity. With timewarrior, I can track a period with the ":adjust" hint that would insert the new event and split the existing event into two events before and after the new event.

If I were to use timetagger, this turns into many more steps. This is probably the easiest I can think of.

1. Insert the new event and ignore that it overlaps.
2. Adjust the overlapped event to end before the new event. Note the original end time.
3. Recreate the overlapped event to start after the new event and set the original end time. A "clone" button would be nice here.

This can be potentially be implemented with #347 by including the split/adjust option rather than stopping the insert.

I like the timetagger UI but I feel its UX could be improved a little bit with split/duplicate/insert/merge functions. It's very mouse-oriented. I might check out the CLI to see if it can be extended to do what I need.