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In addition to being new enough it needs to have NSS support built in

...entering the pwd for the certificate store

Also Move CertificateStoreImpl to PopplerCertificateStore, since
PopplerCertificateStore was declared but never defined and use it
in CertificateTools to get the certificate list instead of calling
the poppler classes directly

Makes more sense

This way we don't use nullptr as part for the dialogs, it's always a bit
ugly to do so

And remove from toolbar, it's not that important it should be there

Also increase version

 * Don't make WidgetAnnotation know about signatures stuff, widget
   annotations are for multiple things
 * Don't create an "empty" widget annotation and then call sign on it
   (which is wrong because widget annotations can be multiple things),
   just say sign the document with this data (cert, l&f, etc)
 * Remove the "management" functionality from CertificateTools it was
   only visual, i.e. it didn't really add/remove certificates
 * Ask for the NSS password (if needed)

and fix a leak meanwhile ^_^

Attempting to fix the whole chain of commits is not worth it

- make explicit we're using static QFile func here
- QFile::rename doesn't work across filesystems, so instead
  create it alongside. Might fail if dir is not writeable
- can't use QSaveFile, since poppler sign() func wants a filename

Move certificate config down into generator code
Read available signing certificates from poppler NSS

WIP - add interfaces and methods to get access to NSS-via-poppler
sign certificates, and provide them for signing documents.

Now poppler offers signing via PDFConverter class. Add some error
handling, and store signature in temp file. Document gets reloaded
after replacing original file.