Export of book list as .bib file

[silvia-ivanova-github-rep]

Description

• Exporting a .bib data for all listed books from a selected shelf. The data contains book data and notes for each book. The exported file type is BibTeX.

• If a shelf contains no books, then a message with further information is provided to the user.

• Implemented external storage permission requests in order to be able to save the file.

• The exported file is stored in the "Download"-folder of the device. A message with the path is also shown.

Affects Book and Shelf fragments.

Notes for Reviewer You can export a .bib-file with data for all listed books from a selected shelf.

Please review & comment/approve.

[LuisMossburger]

We still have some merge conflicts - please resolve all of them first.