Add "New Tab" button for Native macOS Tabs

Adds responders for `newWindowForTab` to `AtomApplicationDelegate` and
`NativeWindowMac`, so that `BrowserWindow`s with a `tabbingIdentifier`
will get the new tab button, and both `app` and `window` will emit a
`new-tab-for-window` event.

atom/browser/native_window_observer.h

@@ -78,6 +78,7 @@ class NativeWindowObserver {
   virtual void OnWindowLeaveHtmlFullScreen() {}
   virtual void OnTouchBarItemResult(const std::string& item_id,
                                     const base::DictionaryValue& details) {}
+  virtual void OnNewWindowForTab() {}
 
   // Called when window message received
   #if defined(OS_WIN)