Nearly working shape caching

This implements caching of shapes using QPixmapCache following the
design of QGraphicsView's caching, with some flake-specific changes.
(Flake can show different sets of shapes on different canvases, while
QGraphicsView shows the same scene on different paint devices.)

Current problems:

* when editing, shapes get overpainted incorrectly
* in kword, the margin offset seems wrong, so the page is broken
off at the wrong place.
* editing text doesn't work atm