Alias "unknown" message types to "none"
Alias "unknown" message types to "none"

One can use the "unknown" message type to flush the bus from all the
queued messages. To make this more intuitive we alias the unkown message
to "none".

RidgeRun/gstd-1.x,RidgeRun/gstd-1.x,RidgeRun/gstd-1.x,RidgeRun/gstd-1.x