Improved limit()/clamp() behaviour - now it doesn't call nor(), reusing the
(needed anyway) len var, resulting in *one* scl() call instead of *two*.
Also, we use cached len, so that we don't have to calculate it twice.
Mostly beneficial for Vector3 class.
Also adds limit2(),setLength(), setLength2() and removes docs inherited
from Vector interface
fixes missing keycodes, cleans up & sorts existing ones - an essential/prerequisite commit for further Input improvements, e.g. https://github.com/libgdx/libgdx/issues/2524