Release version 40.rc

The source data for this is (skipping the Translations):

* Bugs fixed:
 - #1131 New Carousel
 - #1132 Duplicate add-ons shown for VLC
 - #1139 Version history: Followup issues
 - #1141 flatpak: Remove of a repository doesn't remove it from the GUI
 - #1142 GsApplication: Crash when run as root
 - #1145 Release date not visible in version history
 - #1147 Refactor icon/pixbuf handling
 - #1148 Refactor app key colour generation
 - #1151 flatpak: Not having latest commit is not a problem
 - #1158 Missing context for permission and license strings
 - #1159 Improve carousel button appearance on non-standard themes
 - #1161 LicenseRef-proprietary only recognised case-sensitively
 - !625 Minor README and RELEASE.md fixes
 - !628 Honor G_MESSAGES_DEBUG env var
 - !629 Update libhandy requirement
 - !630 build: Bump plugin API version for recent GsApp API changes
 - !631 GsDebug improvements
 - !633 Make add-on rows not selectable
 - !637 gs-folders: Remove some apps from Utilities folder
 - !638 Add git commit to version in non-release builds
 - !643 gs-plugin-loader: Reset online-updates-timestamp only when did the update

By the way, running appstreamcli metainfo-to-news regenerates the NEWS file quite a lot here. Maybe it's due to having used AppStream version 0.14.1, I do not know. For the comparison, the diff is here. It won't be a problem, but it just adds more places where one can make things wrong during the release process. I do not know whether it's caused by previous manual file editing or different versions of the appstreamcli producing different format.