add source location/URL to Qiqqa Library record & update source location/URL for an existing PDF

[GerHobbelt]

v79/v80 Qiqqa only records local file paths as source locs, but it is very handy to have the source URL for any PDF.

About updating that URL: I regularly import PDFs from my local Download directory, but when I go to the Qiqqa Sniffer, I might want not just want to grab the BibTeX from there, but also the source URL of the PDF.

Getting the source loc info out of Qiqqa is another matter still...

[GerHobbelt]

Done as per #33.

Relevant commits (there MAY very well possibly be additional relevant edits in other commits; I wanted this done fast, so it's not pedantic pullreq-Tauglich neat):

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Revision: 4c3b1ed

• fix crash in PDF import when website/webserver does not provide a Content-Disposable HTTP response header
• add ability to cope with PDF links, e.g. when a HTML page is shown with PDF embedded instead of the PDF itself
• detect PDF files in URLs which have query parameters: '.pdf' is not always the end of the URL for downloading the filename

Revision: 6283d89
work being done on #29 + #30: augmenting our Jolly Sniffer.

Revision: a80be7d
done work on #27 and on the lockups of Qiqqa (some critical sections in there were humongous in both code side and run-time duration; now the number of lock-ups due to very slow loading PDFs coming in from the Qiqqa Sniffer should be quite reduced: work related to #18

Revision: 84fe992
feature added: store the source URL (!yay!) of any grabbed/sniffed PDF. Previously the source path of locally imported (via WatchFolder) PDFs was memorized in the Qiqqa database. It adds great value (to me at least) when Qiqqa can also store the source URL for any document -- this is very handy information to have as part of augmented document references!)

[GerHobbelt]

Closing ^{and decluttering the issue list so it stays workable for me}: fixed in https://github.com/GerHobbelt/qiqqa-open-source mainline=master branch, pending #15 / any maintainer rights/actions.