Make story mode point system clearer

[supertuxkart-sourceforge-migration]

Author: auria

Quite a few people were confused that when you complete a medium challenge after completing it at easy, or completing it hard after completing it at medium, you only get one point. The text on screen says "this trophy is worth X points", perhaps we should make it say something like "this trophy is worth X more point that your preview trophy for the same challenge"

Migrated-From: https://sourceforge.net/apps/trac/supertuxkart/ticket/1148

[supertuxkart-sourceforge-migration]

Author: auria The strings have been changed to be a little clearer, so this is better now