Async loading complex project causes viewer crash on shutdown

The issue is an assert in QRawFontPrivate::setFontEngine(), which checks that the engine is set and cleared in the same thread. Short investigation into issue reveals that fonts are registered for each presentation when building the scene, which is pretty much a waste of time anyway, since all presentations register the same font folder. When async loading subpresentations, the scene is built in a worker thread, leading to registering fonts in worker thread.

The font registration should be centralized to engine instead of doing it in the scene. This should also help a bit with startup time, as duplication is removed.