move PpsView's state into document model

Currently, PpsView handles both input (gestures, keyboard navigation) and output (rendering), as well as the state. In the spirit of model–view–* and especially model–view–view model separation.

Being able to refactor the state handling and to refactor the actual UI part independently will simplify #7. Additionally, having all document related code in one area is a prerequisite for #104. Furthermore, without this, moving tabs between windows for #54 would not be possible.