When plugins have not been scanned, and a session with
missing plugins is opened, ask to scan plugins.
This also consolidates translatable strings with plugin-selector.
Copyright-holder and year information is extracted from git log.
git history begins in 2005. So (C) from 1998..2005 is lost. Also some
(C) assignment of commits where the committer didn't use --author.
Robin Gareus
Thomas Brand
Paul Davis
Carl Hetherington