tasks widget hacking

[Christian Mollekopf]

  i moved all the bookkeeping to the TaskGroupItem, so the main interface  
to the lib is now there.
  As soon  an item enters a group the AbstractTaskItem for the task/group  
is created and show,
  when it leaves the group, it's destroyed again. This way we should have a  
quite clear structure.

Unfortunately i broke somehow the splitGroup. Items are not removed  
properly when collapsing a split group.

Remaining issues are for instance the ones with the QGraphicsGridLayout.

i believe lately the stretch factor was not always correctly applyed on  
expanded groups
when there wasn't a second row to force the grid columns into the correct  
size, although i can't reproduce it right now.

Regards,

Christian