restructure into smaller files & add IWYU/clang-tidy

Signed-off-by: Folling <mail@folling.io>

include/ikarus/folders/entity_folder.h

@@ -0,0 +1,22 @@
+#pragma once
+
+/// \file entity_folder.h
+/// \author Folling <mail@folling.io>
+
+#include <ikarus/id.h>
+#include <ikarus/macros.h>
+
+/// \addtogroup folder Folders
+/// @{
+
+IKARUS_BEGIN_HEADER
+
+/// \brief A entity folder, storing entities and other entity folders.
+struct IkarusEntityFolder {
+    /// \private \brief The id of the entity folder.
+    IkarusId id;
+};
+
+IKARUS_END_HEADER
+
+// @}