Move KMime to KDE Frameworks

Remaining issues:

• Remove the deprecated add/removeContent API (after ensuring all remaining consumers are ported)
• Make Content::parent and Content::topLevel const-correct (needs removal of the MessageCore::NodeHelper iteration methods)
• Make Content::decodedText const (needs the move of the decoded flag from Headers::CTE to ContentPrivate, and for that potentially new API to indicate whether you set a decoded or encoded content on a KMime::Content node)
• DateFormatter probably should move out to its single use, which would remove the dependency on translations even.
• Remove or change Content::defaultCharset(), that doesn't return UTF-8 yet...
• Replace Header::from7Bit overloads with a single one taking a QByteArrayView
• Move to automatically generated forwarding includes.
• Fix Windows unit test and require passing unit tests on all platforms.
• Adjust to KF naming and versioning (in a branch?) and go through the KF review process
• Timing: aim for right after 24.08 branching (or failing that, right after next Gear branching)