[Functionality request] Prevent user from inserting a clef into a syllable when the clef is at the beginning of a staff

If a user tries to insert a `clef` element into a syllable, we could make it so that Neon would check that that clef is in the middle of a syllable. If not, Neon would block the action, notifying the user that they can only insert a clef into a syllable when that clef is in the middle of a syllable.

See #1243 for details on why we decided this would be a useful feature.

Start by reading issue #1243 for the rationale, then trace the existing clef-insertion handling in Neon. The change is done when insertion is blocked and the user is notified for a clef at the beginning of a staff, while insertion remains available when the clef is in the middle of a syllable.

Written by the indexing model from the issue text.