#2850 MirrorPropertyTreeWebsocket - Add commands addListener, set, get, and exec

[Pat Callahan]

In order to use the MirrorPropertyTreeWebsocket in a client similarly to the way one would in PropertyChangeWebsocket, the Mirror is being updated to process addListener, get, set and exec commands. The MirrorPropertyTreeWebsocket code is more robust than PropertyChangeWebsocket and solves some of the issues seen there.

The change will allow users to set up a client with any list of properties. It is not expected to break any existing usage of MirrorPropertyTreeWebsocket.