Add an option to fold pre-release release notes into the new release

Volker Krauserequested to merge
vkrause/appstream-metainfo-release-update:work/vkrause/snapshot-folding into master

The idea is that release notes added for CD builds will be folded into the "real" release notes as soon as those releases are created. This matters for applications using pre-release release notes to communicate with CD master build users.

See frameworks/kcoreaddons!550 (merged) for the bigger picture.

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