Do not crash in an empty symmetric IncrementalPlot

This crash could be reproduced by starting QGroundControl, switching to
plot, then clicking the "Symmetric" checkbox.

Sensor Hil for flightgear

Quadrotor HILS Update

This commit disabled RC RAW signal callback.

Fix toolbar battery voltage not updating

Implement message driven receiver pairing

Miscellaneous Windows QGC build fixes.. including APM crash fix

- APM resources were not installed

- DLL copying belongs in qgroundcontrol.pro, not qupdate.pro
- Turn on link time code generation for better (global) optimization

update master docs (first visible on github) to explain how to grab QGC2

Fix Spektrum pair buton such that it works every time.