Fix regression on menu validation

-validateMenuItem: takes precedence over -validateUserInterfaceItem:
and would make the latter useless. Fixed by merging the former into
-validateUserInterfaceItem:.

MacDown/Code/Document/MPDocument.m

@@ -481,20 +481,15 @@ - (BOOL)validateUserInterfaceItem:(id<NSValidatedUserInterfaceItem>)item
         BOOL validness = !self.previewVisible && self.previousSplitRatio >= 0.0;
         ((NSMenuItem *)item).hidden = !validness;
     }
+    else if (action == @selector(toggleTOCRendering:))
+    {
+        NSInteger state = self.rendersTOC ? NSOnState : NSOffState;
+        ((NSMenuItem *)item).state = state;
+    }
     return result;
 }
 
 
-#pragma mark - NSMenuValidation
-
-- (BOOL)validateMenuItem:(NSMenuItem *)menuItem
-{
-    if (menuItem.action == @selector(toggleTOCRendering:))
-        menuItem.state = self.rendersTOC ? NSOnState : NSOffState;
-    return YES;
-}
-
-
 #pragma mark - NSTextViewDelegate
 
 - (BOOL)textView:(NSTextView *)textView doCommandBySelector:(SEL)commandSelector