Patch: display tiles/month equivalent for km/h

[neroden]

This applies only *after* the "time-3" patch in this topic: http://forum.simutrans.com/index.php?topic=6629.new#new

This displays the tiles/month equivalent for 100 km/h in the goods list window (underneath the speedbonus information).
I tried to do it in a "clean" manner, so there are some random cleanups in the code (using symbolic constants instead of raw numbers) as well.  This turned out to be easier than I expected.

I haven't sat down and timed a convoy yet (50 km/h convoy over long straight 50km/h road, perhaps).  Other than that it appears to work -- any computational errors should be fixable with the same structure.

EDIT: For James Petts, the adaptation of this to experimental is on the experimental-speed-trans branch on github.

[neroden]

HEY!  SIMUTRANS DEVELOPERS!

Like with the "time-3" patch in the other thread, this has apparently been ignored for two months.  Could someone please take a look.

[neroden]

Updated patch.

[prissi]

Not me before June.

[neroden]

Updated patch, applies against head plus "time-5" patch.

EDIT: updated to apply against head plus "time-6" patch, thanks to discovering that I lost bits of the patch in time-5 (time-6 seems good).