Augment Dialog Service to allow chained dialogs.

As an application developer, I would like to use the dialog service in the following way:

(1) launch a dialog to ellicit user information..

• e.g. pick an item M

(2) launch a second dialog (from pressing OK on the first) with item M as the context

• to allow the user to select further options for item M

In the current implementation, this is not yet possible.