Help+LibManual: Move non-UI-specific manual handling to LibManual

This is a first step in deduplicating code within and across Help and
man.

Because LibManual also doesn't contain any DeprecatedString, some
adjustments to Help's string handling is included, just to interoperate
with LibManual better. Further work in this area mostly requires String
APIs in LibGUI.

Userland/Libraries/LibManual/PageNode.h

@@ -0,0 +1,38 @@
+/*
+ * Copyright (c) 2019-2020, Sergey Bugaev <bugaevc@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/NonnullRefPtr.h>
+#include <LibManual/Node.h>
+
+namespace Manual {
+
+class SectionNode;
+
+class PageNode : public Node {
+public:
+    virtual ~PageNode() override = default;
+
+    PageNode(NonnullRefPtr<SectionNode> section, String page)
+        : m_section(move(section))
+        , m_page(move(page))
+    {
+    }
+
+    virtual NonnullRefPtrVector<Node>& children() const override;
+    virtual Node const* parent() const override;
+    virtual String name() const override { return m_page; };
+    virtual bool is_page() const override { return true; }
+
+    ErrorOr<String> path() const;
+
+private:
+    NonnullRefPtr<SectionNode> m_section;
+    String m_page;
+};
+
+}