GCS Control via GCSReceiver

This makes the GCS Controller gadget have an option to control the UAV via the GCSReceiver object instead of overriding ManualControlCommand.  This is useful, especially for simulation, where you can act as if there is a real transmitter.

In addition SDL is now optional.  If you want to compile with SDL type

   make GCS_QMAKE_OPTS="CONFIG+=SDL" -j7 gcs

as is, SDL is not used by default.  I can happily swap the defaults, but I don't think many people are using USB joysticks anyway.