Move CSFML source files to CSFML/ directory

This helps make it easier to distinguish SFML files from CSFML files.
This is especially important for CSFML private headers which are
often C++ code with .hpp file extensions. This change means SFML and
CSFML can use headers with the same name without conflict.

doc/mainpage.hpp

@@ -12,8 +12,8 @@
 ///
 /// \code
 ///
-/// #include <SFML/Audio.h>
-/// #include <SFML/Graphics.h>
+/// #include <CSFML/Audio.h>
+/// #include <CSFML/Graphics.h>
 ///
 /// int main()
 /// {