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.

test/System/Sleep.test.cpp

@@ -1,4 +1,4 @@
-#include <SFML/System/Sleep.h>
+#include <CSFML/System/Sleep.h>
 
 #include <catch2/catch_test_macros.hpp>