restructure into smaller files & add IWYU/clang-tidy

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

include/ikarus/objects/entity.h

@@ -0,0 +1,41 @@
+#pragma once
+
+#include <ikarus/id.h>
+#include <ikarus/macros.h>
+
+/// \file entity.h
+/// \author Folling <mail@folling.io>
+
+/// \defgroup entities Entities
+/// \brief Entities are the core building blocks of Ikarus.
+
+IKARUS_BEGIN_HEADER
+
+/// \brief Entities are the core building blocks of Ikarus.
+/// \detials Blueprints and Properties define the structure of the data.
+/// Entities define the data itself.
+///
+/// Properties can be associated with Entities in two ways:
+/// - Directly: The property is linked to the entity.
+/// - Indirectly: The property is linked to a blueprint of the entity.
+///
+/// For each property an entity is linked to, it has a value. These values depend on the property's type.
+/// For more information on the types see the property documentation.
+///
+/// Values are the core type of data within Ikarus.
+/// Each value is associated with one page and one property.
+///
+/// \remark Values are typed, the type of a value is specified by its associated property.
+/// For more information on the types see the property documentation.
+///
+/// \remark Values are guaranteed to be in valid format for a given type
+/// but not guaranteed to be valid under the settings of the property.
+/// This is because changing the settings can invalidate existing values without resetting them.
+struct IkarusEntity {
+    /// \private \brief The ID of the entity.
+    IkarusId id;
+};
+
+IKARUS_END_HEADER
+
+// @}