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.mm

@@ -0,0 +1,20 @@
+/*
+ * Copyright (c) 2023, Nico Weber <thakis@chromium.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#import "MacPDFWindowController.h"
+
+@implementation MacPDFWindowController
+
+- (instancetype)initWithDocument:(MacPDFDocument*)document
+{
+    if (self = [super initWithWindowNibName:@"MacPDFDocument" owner:self]; !self)
+        return nil;
+
+    return self;
+}
+
+
+@end