Move document parsing into Service

[TomTasche]

All the work currently handled by *Loader classes inside of DocumentFragment should move to a Service.

Two goals I hope to achieve by doing so:

1. make the app more resilient: especially the PDF library we use crashes a lot (in C++). We could catch such crashes if they were running in a separate process (see this blog post)
2. clean support for orientation changes: at the moment we are supporting orientation changes using a dirty hack: retained fragment instances. Using a Service would allow us to restart the UI without stopping the heavy work in the background (document parsing).