Refactored FlightMenu, minor fixes

-corrected some residual comments referencing bearing instead of heading
-corrected setHeadingSize using bearingSize as a parameter instead of
headingSize
-added/improved some commenting
-removed some bracketing where meaning is not lost to improve
readability
-refactored multScale(), normScale()
+parameter order changed -> pos, min, max
-added function constrain() to reduce code duplication, improve
readability
-fixed setInput() to properly deal with being passed null values
-optimised mousePressed()
+now chains else ifs