Move everything under the guecs/ directory in src/ so that it meshes better with other projects.

tests/shaders.cpp

@@ -1,7 +1,7 @@
 #include <catch2/catch_test_macros.hpp>
 #include <fmt/core.h>
 #include <string>
-#include "sfmlbackend/shaders.hpp"
+#include "guecs/sfml/shaders.hpp"
 
 using namespace fmt;