SpaceAnalyzer: Remove an unnecessary level of inheritance

The TreeMapNode and TreeMap structs inside TreeMapWidget.h both had
single implementers, TreeNode and Tree inside main.cpp. The indirection
was removed and the new structures were moved to their own file

Userland/Applications/SpaceAnalyzer/Tree.h

@@ -0,0 +1,45 @@
+/*
+ * Copyright (c) 2022, the SerenityOS developers.
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/DeprecatedString.h>
+#include <AK/Forward.h>
+#include <AK/OwnPtr.h>
+#include <AK/RefCounted.h>
+#include <AK/Vector.h>
+
+struct TreeNode final {
+    TreeNode(DeprecatedString name)
+        : m_name(move(name)) {};
+
+    DeprecatedString name() const { return m_name; }
+    i64 area() const { return m_area; }
+    size_t num_children() const
+    {
+        if (m_children) {
+            return m_children->size();
+        }
+        return 0;
+    }
+    TreeNode const& child_at(size_t i) const { return m_children->at(i); }
+    void sort_children_by_area() const;
+
+    DeprecatedString m_name;
+    i64 m_area { 0 };
+    OwnPtr<Vector<TreeNode>> m_children;
+};
+
+struct Tree : public RefCounted<Tree> {
+    Tree(DeprecatedString root_name)
+        : m_root(move(root_name)) {};
+    ~Tree() {};
+    TreeNode m_root;
+    TreeNode const& root() const
+    {
+        return m_root;
+    };
+};