This squashes the following commits:

- gpg4libre: share static xmlsec lib between nss and gpg
- gpg4libre: fix gpg signature generation
- gpg4libre: initial GPG signature validation
- gpg4libre: Make signature dialog work with two signing services
- gpg4libre: Having this dllpublic should be fine now since it's in inc/
- gpg4libre: fix build, explicit ctor call
- gpg4libre: now use the gpg security env
- gpg4libre: [API-CHANGE] add certificate kind (X509 vs. OpenPGP)
- gpg4libre: List both (x509 and gpg) existing signatures
- gpg4libre: Init xmlsec in one place before creating the gpg/x509 services
- gpg4libre: write PGPData info, get more metadata out for gpg key
- gpg4libre: Fixup unit tests, now that SecurityContext is needed
- gpg4libre: Make viewing signatures work for gpg signatures
- gpg4libre: some code improvements, add metadata for OpenPGP keys
- gpg4libre: make signature impl swappable in-situ during validation
- gpg4libre: actually take key from user selection
- gpg4libre: fix build for windows and mac

Change-Id: I3e36b22cefba4c6195bcf8b85b3f7a2cc101b845
Reviewed-on: https://gerrit.libreoffice.org/39076Reviewed-by: Katarina Behrens <Katarina.Behrens@cib.de>
Reviewed-by: Thorsten Behrens <Thorsten.Behrens@CIB.de>
Tested-by: Thorsten Behrens <Thorsten.Behrens@CIB.de>