GdkWindow -> GdkSurface: File renames

GdkWindow -> GdkSurface: File renames

Rename all *window.[ch] source files.

This is an automatic operation, done by the following commands:

for i in $(git ls-files gdk | grep window); do
    git mv $i $(echo $i | sed s/window/surface/);
    git sed -f g $(basename $i) $(basename $i | sed s/window/surface/) ;
done

git checkout NEWS* po-properties po