Human time display

Events that start and end on the same human day are displayed without end date, just time.
This is how most humans would write it.
Example (in german locale):
Freitag, 25. April 2008 - 22:00 ... 4:00

I also did some extensive refactoring of the theme functions, so this display formnat can be set in ONE place for all modes (loads of code duplication before).

I hacked this for our party-calendar at www.das-z.de
Hope you have as much fun with it as us.

This is part of #249045: Human display compound patch