Reduce angle drift by adding level correction to the estimator

Leverage a priori knowledge that the drone's average attitude is level.
Explanation: https://t.me/opensourcequadcopter/158.

gazebo/flix.h

@@ -27,6 +27,7 @@ void step();
 void computeLoopRate();
 void applyGyro();
 void applyAcc();
+void applyLevel();
 void control();
 void interpretControls();
 void controlAttitude();