Associating (new) URL with bibliography when saving

When saving a newly created bibliography to a file, set the file's
URL as a property to the bibliography object.
The URL stored in this property allows, among others, to recognize
URLs associated with individual entries, such as PDF files, as
relative to the BibTeX file. Without an URL, only absolute URLs
would be possible.

This commit is a manual forward-port of commit 750915c3
from branch 'kbibtex/0.10' which in its turn is a forward-port of
commit bde43759 from branch 'kbibtex/0.9'.

CCBUG: 409281