Make shift key invert the "open in background" preference

- Also add an appropriate menu item.

Mac/MainWindow/MainWindowController.swift

@@ -179,6 +179,10 @@ class MainWindowController : NSWindowController, NSUserInterfaceValidations {
 	public func validateUserInterfaceItem(_ item: NSValidatedUserInterfaceItem) -> Bool {
 		
 		if item.action == #selector(openArticleInBrowser(_:)) {
+			if let item = item as? NSMenuItem, item.keyEquivalentModifierMask.contains(.shift) {
+				item.title = Browser.titleForOpenInBrowserInverted
+			}
+			
 			return currentLink != nil
 		}