* Items are now in defined z-order: link (-1) -> component (0) -> node (1)
* Ensures that nodes are on top of things
* Makes sure itemAt() works properly on mouse release
* Code cleanup: removed superfluous events and redundant code


git-svn-id: https://svn.origo.ethz.ch/fidelity@27 53d1999f-d1a8-4366-aa61-588fded17473