restructure into smaller files & add IWYU/clang-tidy

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

include/ikarus/folders/property_folder.h

@@ -0,0 +1,23 @@
+#pragma once
+
+#include <ikarus/id.h>
+#include <ikarus/macros.h>
+
+/// \file property_folder.h
+/// \author Folling <mail@folling.io>
+
+/// \addtogroup folder Folders
+/// @{
+
+IKARUS_BEGIN_HEADER
+
+/// \brief A property folder, storing properties and other property folders.
+/// \remark Property folders are scoped to the blueprint or entity they are associated with.
+struct IkarusPropertyFolder {
+    /// \private \brief The id of the property folder.
+    IkarusId id;
+};
+
+IKARUS_END_HEADER
+
+// @}