[A11y] Emit object:property-change:accessible-value events where appropriate

An object:property-change:accessible-value event would be helpful when the user moves the point at which an audio event is to be cut. Right now an AT such as Orca has no way to identify that this is taking place, let alone present the new location to a user who is blind.

I think such an event would also be nice when the user manually moves the position of the timeline. (I do *not* think this event would be desirable when the position moves while the user is recording because it would result in way too many events that we almost certainly do not care about anyway.)