#12 Upgrade sheets to be built on QGraphicsView

[Nathan Carter]

(This is probably no longer relevant, since so much more updating needs to be done on GE than just this, but I preserve this ticket here in my transition from Trac to Allura.)
My current implementation of sheets is several years old, before QGraphicsView (in Qt4) was invented. That implementation is much better than mine and would simplify my code a lot and improve the user experience. But converting it to use QGraphicsView would involve a nontrivial amount of rewriting and testing, so it was not done as part of the Qt3->Qt4 port of GE.