Add pdf.js

Known issues that needs to be fixed:

1. Visual glitches: page counter and zoom combobox (too large)
2. viewer layout needs to changed as requested by pdfjs
3. locales needs to added to pdfjs resource
4. Security indicator is broken
5. Translations are not working
6. PDF page is not loaded after browser restart
7. print buttons does not work: https://bugs.webkit.org/show_bug.cgi?id=202363
8. navigation buttons are sometimes "screwed" (use duckduckgo.com and visit pdf, press back and you are in a loop) (site specific issue, accepted)
9. after switching to EphyDownload files are not loaded 100% due to non existing files after finished signal is emitted

Solutions:

@5. translation works as soon as base_uri is set in webkit_web_view_load_alternate_html but then the URL is changed and back/forward leads in ephy-resource:///...

@9. bug is somewhere in webkit, solution: switch back to libsoup solution for the moment.

Implementation detail:

Due to CORS pdf is loaded and added in base64 format to generated html page. Inspector cannot handle large html files and freezes...

@exalm Do you have some time to fix the visual glitches?