Horizon Testing

Quick test of the Horizon Mode refactoring PR12231 (which includes the new fast angle mode). Tested on default values (no levelling if sticks are more than 75% out, or angle more than 150 degrees), with a highly responsive little 3" race type quad, in a tight little park.
Details: github.com/bet....
Horizon mode adds a self-levelling factor to acro inputs when the sticks are closer to centre and the quad is close to level. Flips and rolls are easy. The only 'problem' is when exiting a big slow roll 'steadily and slowly', the acro rate gets levelling added to it, which flattens it out, and the flatter it gets, the more it wants to level out. So it kind of 'pulls you back' more strongly than an acro pilot would expect, unless you kind of 'drive' it around by slowly easing the stick back to centre. In this case the levelling strength was quite high, leading to a bit more aggression in returning to level than I would have liked. That's the main trade-off with Horizon: smoothness returning to centre vs levelling strength.
The overall 'horizon_level_strength' factor could be pulled back a bit, for someone more accustomed to Acro like me. It is deliberately strong by default to suit beginners. Horizon is difficult to fly really smoothly unless the entering is made fairly weak; Acro is definitely much smoother. But the self-levelling would appeal to a lot of people, and it can be adjusted to happen only around centre sticks, and to be strong or weak...
I could do inverted hangs on defaults, but f the max angle was set to say 120 degrees, it would be easier.