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).
Remove assert(is_request)

Assert causes crash when we try to handle reply
for M7, M8 and M9 commands that are sent from optional senders.
Optional handlers CanHandle is called and if handler is Request
handler assert is triggered.
Implement handling of play / pause / teardown

This patch adds possibility to control playback
state of MiracGstSink and uses that functionality
in sink state machine.