Add support for specifying fixed RTK base position

This does the following:
- add RTCMMavlink class that streams RTCM messages via mavlink
- add GPSProvider class that opens a serial device and uses the gps driver
  submodule to get an RTCM stream
- add a GPSManager class that manages the RTCMMavlink & GPSProvider classes
- implement gps driver callback & definitions using Qt
- add the GPSManager to the QGCToolbox

To test this, uncomment the _gpsManager->setupGPS("ttyACM0"); in
QGCToolbox.cc