AK: Move Handle from LibCore and name it MaybeOwned

The new name should make it abundantly clear what it does.

AK/MaybeOwned.h

@@ -0,0 +1,60 @@
+/*
+ * Copyright (c) 2022, Tim Schumacher <timschumi@gmx.de>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/NonnullOwnPtr.h>
+#include <AK/Variant.h>
+
+namespace AK {
+
+template<typename T>
+class MaybeOwned {
+public:
+    template<DerivedFrom<T> U>
+    MaybeOwned(NonnullOwnPtr<U> handle)
+        : m_handle(adopt_own<T>(*handle.leak_ptr()))
+    {
+    }
+
+    // This is made `explicit` to not accidentally create a non-owning MaybeOwned,
+    // which may not always be intended.
+    explicit MaybeOwned(T& handle)
+        : m_handle(&handle)
+    {
+    }
+
+    T* ptr()
+    {
+        if (m_handle.template has<T*>())
+            return m_handle.template get<T*>();
+        else
+            return m_handle.template get<NonnullOwnPtr<T>>();
+    }
+
+    T const* ptr() const
+    {
+        if (m_handle.template has<T*>())
+            return m_handle.template get<T*>();
+        else
+            return m_handle.template get<NonnullOwnPtr<T>>();
+    }
+
+    T* operator->() { return ptr(); }
+    T const* operator->() const { return ptr(); }
+
+    T& operator*() { return *ptr(); }
+    T const& operator*() const { return *ptr(); }
+
+private:
+    Variant<NonnullOwnPtr<T>, T*> m_handle;
+};
+
+}
+
+#if USING_AK_GLOBALLY
+using AK::MaybeOwned;
+#endif