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/table/settings.ini

@@ -42,6 +42,7 @@ static bool InvalidateCompanyInfrastructureWindow(int32 p1);
 static bool InvalidateCompanyWindow(int32 p1);
 static bool ZoomMinMaxChanged(int32 p1);
 static bool MaxVehiclesChanged(int32 p1);
+static bool ImprovedBreakdownsSettingChanged(int32 p1);
 
 #ifdef ENABLE_NETWORK
 static bool UpdateClientName(int32 p1);
@@ -1143,7 +1144,7 @@ from     = SL_IB
 guiflags = SGF_NO_NETWORK
 def      = false
 str      = STR_CONFIG_SETTING_IMPROVED_BREAKDOWNS
-
+proc     = ImprovedBreakdownsSettingChanged
 
 ; station.join_stations
 [SDT_NULL]