Isolate backend running code from the rest of the application

There has been repeated discussions on how to deal with the fact that many document formats that Papers (and Evince) open are opaque blobs. This has security implications, but also other consequences. The most remarkable one is that Papers and Evince are multi-instance applications. Every document is opened in a different process to avoid one document crashing bringing down with it the complete application. To fix it, it would be necessary to wrap the code responsible for the backends into something like bwrap or equivalent.