Proposal: Add OldValue to DataContextChangedEventArgs

[bjorn-malmo]

When DataContextChanged occurs, it would be really helpful to be able to access to old datacontext, since Unloaded/Loaded events cannot be used to manage state (these appears to be async and not deterministic in order. See Issue 1900)

Rationale

In order to manage state and resource allocation within ViewModels, it would be helpful to have access to the unloaded Datacontext. The workaround, to have a local reference to the current datacontext, adds unnecessary code and complexity.

This would be completely backwards compatible and probably quite easy to implement.