Remove hardcoded dep of OdtDocument code on op UpdateMetadata

Following initial design there should be multiple different operation sets possible, all working on the same document classes.
So relying on a certain optype and its spec in the code of ops.OdtDocument is conflicting with that.

This patch means that for OpUpdateMetadata there will be now two separate signals emitted. I do not see a problem with that, as this op is not done often.

Ran across this when drafting that other signal-only-after-op-execution-completed PR, still not done.