[API] Expose an element's children

[XiaoMigros]

Resolves: #28419

Since #28957 it's easy to expose this to the API. Testing this a few months ago when std::list was used led to some unstable results, but now that it's an std::vector like the other plugin list properties I haven't encountered any issues.

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[cbjeukendrup]

The problem with children is that it returns all elements whose parent is this, but those are not necessarily actually part of the score (for example, they might be in the undo stack). So therefore I'm not sure whether we should expose it.