MacPDF: Introduce MacPDFWindowController

I'd like to add a sidebar, and NSSplitViewItem apparently isn't
accessibly in .xib files without contortions. So I want to move
to creating the window in code, and this is a step towards that.

No behavior change.

Meta/Lagom/Contrib/MacPDF/MacPDFWindowController.h

@@ -0,0 +1,21 @@
+/*
+ * Copyright (c) 2023, Nico Weber <thakis@chromium.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include "CocoaWrapper.h"
+
+NS_ASSUME_NONNULL_BEGIN
+
+@class MacPDFDocument;
+
+@interface MacPDFWindowController : NSWindowController
+
+- (instancetype)initWithDocument:(MacPDFDocument*)document;
+
+@end
+
+NS_ASSUME_NONNULL_END