MacPDF: Add an NSOutlineViewDataSource for the PDF outline

Not used yet.

Meta/Lagom/Contrib/MacPDF/MacPDFOutlineViewDataSource.h

@@ -0,0 +1,28 @@
+/*
+ * Copyright (c) 2023, Nico Weber <thakis@chromium.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include "CocoaWrapper.h"
+
+#include <LibPDF/Document.h>
+
+NS_ASSUME_NONNULL_BEGIN
+
+// Objective-C wrapper of PDF::OutlineItem, to launder it through the NSOutlineViewDataSource protocol.
+@interface OutlineItemWrapper : NSObject
+
+- (Optional<u32>)page;
+
+@end
+
+@interface MacPDFOutlineViewDataSource : NSObject <NSOutlineViewDataSource>
+
+- (instancetype)initWithOutline:(RefPtr<PDF::OutlineDict>)outline;
+
+@end
+
+NS_ASSUME_NONNULL_END