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_window.h

@@ -91,6 +91,7 @@ class Window : public mate::TrackableObject<Window>,
   void OnExecuteWindowsCommand(const std::string& command_name) override;
   void OnTouchBarItemResult(const std::string& item_id,
                             const base::DictionaryValue& details) override;
+  void OnNewWindowForTab() override;
 
   #if defined(OS_WIN)
   void OnWindowMessage(UINT message, WPARAM w_param, LPARAM l_param) override;