Improved breakdowns: Various changes and fixes.

* Revert breakdown_chance to (mostly) its original behaviour.
* Create a new breakdown_chance_factor to hold breakdown_chance
  from improved breakdowns logic.
* Revert airport crash probabilities back to original behaviour, with
  modified behaviour only during emergency landings.
* Low power breakdowns now only reduce the power of the engine which
  has broken down.
* Low power breakdowns no longer reduce speed directly.
* Add callback function to run whenever improved breakdowns setting
  is changed. Reset breakdown_chance_factor where required.
* More whitespace/formatting...

src/vehicle_base.h

@@ -195,6 +195,7 @@ public:
 
 	byte breakdown_severity;            ///< severity of the breakdown. Note that lower means more severe
 	byte breakdown_type;                ///< Type of breakdown
+	byte breakdown_chance_factor;       ///< Improved breakdowns: current multiplier for breakdown_chance * 128, used for head vehicle only
 	SpriteID colourmap;                 ///< NOSAVE: cached colour mapping
 
 	/* Related to age and service time */