Story mode clarifications

[supertuxkart-sourceforge-migration]

Author: auria

Clarify in story mode that the door will open when all challenges have been beaten, some people are a little confused with that.

Also perhaps find a way to point people to the minimap, many people don't seem to realise challenges are shown on the map

[supertuxkart-sourceforge-migration]

Author: sachith500 Hello, I submitted a pull request that adds a message about the door opening after all challenges are completed, to the existing message that players get when they are near a challenge. https://github.com/supertuxkart/stk-code/pull/17 Is there anything I can do to improve this?

[supertuxkart-sourceforge-migration]

Author: sachith500 Sent a pull request which adds a message when a player approaches the doors. https://github.com/supertuxkart/stk-code/pull/23

In addition here's the diff file for the action script in stk-assets\tracks\overworld\scene.xml