[patch] Show required station length in convoi details

[sanna]

It has always been a bit of an annoyance to me that the simplest way to find out how many station tiles that a convoi requires to be fully served has  been to take it to a depot. It seems to me that this is information that should be accessible from the convoi details. I therefore (with my minimal c++ skills) have created a mini-patch based on rev. 2878 that includes the information in this dialogue.

Please note that this patch just makes the convoi details "header" one line longer, and that I am unsure if this can lead to instabilities elsewhere.

I put a screenshot at: http://simutrans-germany.com/files/upload/convoi_details_station_length.png

[VS]

Small but useful improvement - thanks!

[prissi]

I was just working at the odometer, so this will be added right away.