[Sniffer Feature] add options to filter documents list: which are sourced by URL, Local File or not at all (Unsourced)

[GerHobbelt]

So we can go through the set which has their DownloadLocation set up incorrectly / not at all.

[GerHobbelt]

Done as per #33

See also comments at #30 and this commit series:

Revision: 9a7e620
Only when you play with it, you discover what works. The HasSourceURL/Local/Unsourced choices should be OR-ed together as that feels intuitive, while we also want to see 'sans PDF' entries as we can use the Sniffer to dig up the PDF on the IntarWebz if we're lucky. Meanwhile, 'invert' should clearly be positioned off to a corner to signify its purpose: inverting your selection set (while it should probably 🤔 have no effect if a specific document was specified by the user: then we're looking at a particular item PLUS maybe some other stuff?

Revision: 4acc253
Merge branch 'n29'

Revision: 604a5db
Sniffer Features:

• add checkboxes to (sub)select documents which have a URL source registered with them or no source registered at all. ([Sniffer Feature] add options to filter documents list: which are sourced by URL, Local File or not at all (Unsourced) #29)
• add 'invert' logic for the library filter ([Sniffer Feature] add option to INVERT the selected document set (~ "NOT" query operator) #30)

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

[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.

[GerHobbelt]

Related: #27