Sends a command to the client specified in AGUIDTo parameter.
function SendCommandToGUID(Cmd: TRVCmd; AGUIDFrom, AGUIDTo, AGUIDGroup: TGUID): Boolean;
AGUIDFrom must be equal to GUIDMy property of the server (unless you want to simulate a command from another client).
AGUIDTo identifies the addressee (GUIDMy property of the client's receiver)
AGUIDGroup can be used to simulate a command sent to a group of user.
False if the command cannot be sent (for example, this user is not connected), True otherwise.