GCS_MAVLink: initial implementation of MAVLink routing object

will be used to auto-route packets over all available links
GCS_MAVLink: added routing object to GCS_MAVLink
GCS_MAVLink: only call message handler for our own messages
GCS_MAVLink: removed use of mavlink_check_target()

not needed now we do routing properly, as messages will only be
processed if they are for us
Plane: removed use of mavlink_check_target()
AP_Camera: removed use of mavlink_check_target()
AP_Mount: removed use of mavlink_check_target()
GCS_MAVLink: only try to fwd packets to active channels
GCS_MAVLink: added support for a MAVLink snoop function

this can be used to watch messages for different targets
Copter: removed use of mavlink_check_target()
Rover: removed use of mavlink_check_target()
AntennaTracker: use generic mavlink routing

this replaces the special purpose routing in the AntennaTracker
AntennaTracker: removed old PROXY_MODE parameter

this should not be needed any more
GCS_MAVLink: strip out the old GCS_Class class

this is not longer needed as we don't support multiple protocol
classes