The minimum GTK version is now GTK+ 3.4, which is available since 2012.
It's part of Ubuntu LTS 12.04, and so should be available on any halfway
modern linux distribution.
This should allow getting rid of using deprecated GTK+ API much easier.
I merged most of the three separate README files into a single one. I
also assumed that people trying to build OpenClonk from source have a
rough idea of how their build system works, so I removed the tutorial
from there. People who are completely new to this should consult the
wiki or ask on IRC.