Clean up references to MainRootWindow globals

LinkInterface and Vehicle uses mutexes to synchronize writing bytes to the link instead of queued signals

Support up to 64 buttons, not limit to 16
Send a single button command and not a stream of commands at 25Hz
Send button commands through signals (it's coming from a different thread)
Hide gimbal support through analog channels (there are no MAVLink messages to handle it)

Add gimbal to external control signal