Move the RADIO message from ardupilotmega.xml and into common.xml

A comment was made regarding this on the mailing list, and is desirable for my project as well.

Note that in this commit the RADIO message would be moved from its old ID of 166 to a new one of 68. In theory this shouldn't break anyone's code as they should be using the `#defines` generated by MAVLink for all the messages.