New sort order option

Add a new sort option, which should work like DateBk6 for Palm. Items are sorted in the following order:
1) overdue activities
2) acivities due today
3) undated activities
4) dated activities due in the future
This makes sense because undated activities are usually the ones you take care of when you have some spare time.
Implementation should be quite straightforward but would be a considerable improvement in my opinion.