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

tests/guecs.cpp

@@ -1,9 +1,10 @@
 #include <catch2/catch_test_macros.hpp>
 #include <fmt/core.h>
-#include "guecs.hpp"
+#include "guecs/ui.hpp"
 #include <fmt/xchar.h>
 
 using namespace guecs;
 
 TEST_CASE("prototype one gui", "[ecs-gui]") {
+  // accidentally gutted, will recover later
 }