certificate: Replace the delegate with a collection

CMakeLists.txt

src/certificate.h

src/certificatemodel.cpp

src/collectioninterface.cpp

src/collectioninterface.h

src/daemoncertificatecollection.cpp

src/daemoncertificatecollection.h

src/fallbacklocalcertificatecollection.cpp

src/fallbacklocalcertificatecollection.h