Improve PDF worker controller

[mvasilak]

• Adds support for workers, which group works for the same file, so that the relevant web view handler is reused.
• Adds support for different priorities, for using higher priority when extracting text for speech.
• Adds more relevant tests.

[michalrentka]

@mvasilak I went through your latest changes. Looks good. Maybe one note for future if needed. If a preloadedPDFWorkerWebViewHandler is used up in func queue(work: Work, in worker: Worker) -> Observable<Update> it could be nice to pre-load another one (just one preloadPDFWorkerIfIdle() call after line 210 in PDFWorkerController I believe). But if performance is not an issue at this point, it might not be worth it.