MacPDF: Make up and down flip pages when PDF view has focus

Previously, it only reacted to arrow left andn arrow right,
and to opt-up and opt-down.

Meta/Lagom/Contrib/MacPDF/MacPDFView.mm

@@ -164,6 +164,12 @@ static NSBitmapImageRep* ns_from_gfx(NonnullRefPtr<Gfx::Bitmap> bitmap_p)
     [self interpretKeyEvents:@[ event ]];
 }
 
+// Called on down arrow.
+- (IBAction)moveDown:(id)sender
+{
+    [self goToNextPage:self];
+}
+
 // Called on left arrow.
 - (IBAction)moveLeft:(id)sender
 {
@@ -176,6 +182,12 @@ static NSBitmapImageRep* ns_from_gfx(NonnullRefPtr<Gfx::Bitmap> bitmap_p)
     [self goToNextPage:self];
 }
 
+// Called on up arrow.
+- (IBAction)moveUp:(id)sender
+{
+    [self goToPreviousPage:self];
+}
+
 #pragma mark - State restoration
 
 - (void)encodeRestorableStateWithCoder:(NSCoder*)coder