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/api/atom_api_app.h

@@ -117,6 +117,8 @@ class App : public AtomBrowserClient::Delegate,
       bool* prevent_default,
       const std::string& type,
       const base::DictionaryValue& user_info) override;
+
+  void OnNewWindowForTab() override;
 #endif
 
   // content::ContentBrowserClient: