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.h

@@ -252,6 +252,7 @@ class NativeWindow : public base::SupportsUserData,
   void NotifyWindowExecuteWindowsCommand(const std::string& command);
   void NotifyTouchBarItemInteraction(const std::string& item_id,
                                      const base::DictionaryValue& details);
+  void NotifyNewWindowForTab();
 
   #if defined(OS_WIN)
   void NotifyWindowMessage(UINT message, WPARAM w_param, LPARAM l_param);