Added worldmap choosing from command-line and menu.

Added worldmap choosing from command-line and menu.
Contributed maps must be in data/levels/worldmap, included the ones run from the command-line.
It is possible to feed maps with absolute path, but it is just a hack that converts them to relative paths to the worldmap folder. I did this, in order to make it possible for someone to add a mime-type for Konqueror as 'supertux --worldmap' and press a .stwt file from worldmap dir.

About the Contrib Worlds menu, we might consider just making one Contrib menu, and make levels from different folders to create dynamically a map.
Till that day, let's keep this menu.

SVN-Revision: 1555