mixer: complete refactoring & add airmode for yaw

Provides a more generic desaturation algorithm, making it easier to
handle the different mixer strategies.

python reference implementation:
https://github.com/Auterion/Flight_Control_Prototyping_Scripts/tree/master/control_allocation

The behavior for non-airmode and roll/pitch airmode should be the
same as before.

CPU load is slightly higher for the non-airmode case (+0.5%), for airmode
it is the same.