Auto separation: Add setting to scale vehicle lateness adjustments.

No longer set vehicle lateness to 0 if separation fails, instead
leave it as it was.

The setting defaults to 100% (full abruptness, old behaviour).
Reduce the setting if auto separation is too disruptive, e.g. causes
excessive waiting in stations.

Note that this is not savegame compatible.

src/settings_type.h

@@ -440,6 +440,7 @@ struct OrderSettings {
 	bool   no_servicing_if_no_breakdowns;    ///< don't send vehicles to depot when breakdowns are disabled
 	bool   timetable_automated;              ///< whether to automatically manage timetables
 	bool   timetable_separation;             ///< whether to perform automatic separation based on timetable
+	uint8  timetable_separation_rate;        ///< percentage of timetable separation change to apply
 	bool   serviceathelipad;                 ///< service helicopters at helipads automatically (no need to send to depot)
 };