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.cc

@@ -585,6 +585,11 @@ void App::OnContinueUserActivity(
     const base::DictionaryValue& user_info) {
   *prevent_default = Emit("continue-activity", type, user_info);
 }
+
+void App::OnNewWindowForTab() {
+  Emit("new-window-for-tab");
+}
+
 #endif
 
 void App::OnLogin(LoginHandler* login_handler,