Transfer all scripts to .nut files to enable translation

[schirmeier]

In commit bf532ac, @Rusty-Box transferred the Icy Island cut-scene scripts to separate .nut files. This should be done for the other worlds as well, because it enables translation to other languages. For example, currently (both 0.6.3 release and current master) there is no translation for the story/narration in data/levels/world2/forest_intro.stl.

[tobbi]

I think it checks for .stl files as well: https://github.com/SuperTux/supertux/blob/master/makepot.sh#L44

[Zwatotem]

I think it checks for .stl files as well: https://github.com/SuperTux/supertux/blob/master/makepot.sh#L44

@tobbi The .stl files contain scripts in quoted strings, which means _("Text to translate") sequences are escaped like this: _(\"Text to translate\"). I'm not sure, but that may interrupt the work of xgettext tool.

I would still be for script extraction, regardless whether levels get processed, or not, because that makes file organization a little tidier.