Implement group-header layouting

The most tricky part for groups - the layouting - basically works
now for all views (grouping was available only in the icons views
for Dolphin 1.x) and is nearly equally fast as without groups.

Still open:
- Group headers are ugly screen rectangles
- Return valid groups in KFileItemModel instead of the currently
hardcoded testing values
- Dynamically turning on/off groups does not work currently, the
directory must be reentered