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/lang/english.txt

@@ -1400,6 +1400,8 @@ STR_CONFIG_SETTING_TIMETABLE_IN_TICKS                           :Show timetable
 STR_CONFIG_SETTING_TIMETABLE_IN_TICKS_HELPTEXT                  :Show travel times in time tables in game ticks instead of days
 STR_CONFIG_SETTING_TIMETABLE_SEPARATION                         :Use timetable to ensure vehicle separation: {STRING2}
 STR_CONFIG_SETTING_TIMETABLE_SEPARATION_HELPTEXT                :Select whether to ensure separation of vehicles when using automatic timetables
+STR_CONFIG_SETTING_TIMETABLE_SEPARATION_RATE                    :Vehicle separation factor: {STRING2}
+STR_CONFIG_SETTING_TIMETABLE_SEPARATION_RATE_HELPTEXT           :How much of the vehicle separation timetable change to apply at each step
 STR_CONFIG_SETTING_TIMETABLE_SHOW_ARRIVAL_DEPARTURE             :Show arrival and departure in timetables: {STRING2}
 STR_CONFIG_SETTING_TIMETABLE_SHOW_ARRIVAL_DEPARTURE_HELPTEXT    :Display anticipated arrival and departure times in timetables
 STR_CONFIG_SETTING_QUICKGOTO                                    :Quick creation of vehicle orders: {STRING2}