This avoid to parse the same element definition each time user drop the same element in the diagram.
Only the first element build the picture, all other get the created picture which is shared.
2- For use the "implicite shared" QPicture and QPixmap of element, now this isn't the element who build her picture, but a dedicated class who made only this job : build the QPicture and QPixmap of elements.
3- With the two previous novelty, the class CustomElement and GhostElement are became useless, and so was removed.
Some few member function of CustomElement was moved to Element.
git-svn-id: svn+ssh://svn.tuxfamily.org/svnroot/qet/qet/trunk@5492 bfdf4180-ca20-0410-9c96-a3a8aa849046
