The global WFD RTSP message classes re-factoring

This patch drops the unneeded 'TypedMessage' class and encapsulates all the message-related logic within the 'Message' class.
The 'Request' class is a generic 'Message' implementation for the WFD RTSP requests.
Message instances life-cycle management is simplified.

This patch removes also 'WFD' namespace and only the 'wfd' namespace is used within all the dedicated files (the message classes re-factoring made it possible).