MacPDF: Make opt-up and opt-down always go to previous / next page

When the outline has focus, arrow keys navigate the outline instead
of changing the current page.

Add opt-up and opt-down as a way to move by one page even when the
outline has focus. (This matches Preview.app.)

xib change: Added two menu Previous Page with key equivalent opt-up
and Next Page with key equivalent opt-down to Go menu and bound them to
goToPreviousPage: and goToNextPage: on First Responder.

When the outline has focus, the responder chain is outline ->
window, so also add the actions on the window controller, and
let that forward to the PDF view.

Meta/Lagom/Contrib/MacPDF/MacPDFWindowController.mm

@@ -122,6 +122,16 @@
     _outlineView.delegate = self;
 }
 
+- (IBAction)goToNextPage:(id)sender
+{
+    [_pdfView goToNextPage:sender];
+}
+
+- (IBAction)goToPreviousPage:(id)sender
+{
+    [_pdfView goToPreviousPage:sender];
+}
+
 - (IBAction)showGoToPageDialog:(id)sender
 {
     auto alert = [[NSAlert alloc] init];