Keysigs on section break

[sammik]

Resolves: #25168

• On section break add key signatures, and mark them "forSectionBreak".
• When removing section break, remove "for section break" signatures.
• If signature already there, it is preserved (not marked "for section break" and not removed on section break deletion.
• New signatures added to section break are also not marked as "for section break", so not removed on section break deletion.

• I signed the CLA
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