Emit a transact event on the ve.dm.Document when a transaction occurs

We'll need this to let a MetaList observe a Document and adjust to changes.
We should probably also have SurfaceFragment listen to this event instead.

Change-Id: I9e811e242969eb44afe0b4fa8153d0fb1b0071cd
Merge "Emit a transact event on the ve.dm.Document when a transaction occurs"