add file contexts (intent / MIME)

[gekeleda]

Added intents to enable proper android external reader selection, both for epub and pdf, as discussed in #72 and #145 .

I tested it on my Android phone and confirmed it to work as expected, and also checked that it does not break anything for windows (cant open .epub files in windows with external reader, but that was the same before. Opening pdfs in external viewer works)

I would have assigned the issue to me, but I think I don't have the authorisation for that