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/browser.cc

@@ -213,4 +213,11 @@ void Browser::OnWindowAllClosed() {
   }
 }
 
+#if defined(OS_MACOSX)
+void Browser::NewWindowForTab() {
+  for (BrowserObserver& observer : observers_)
+    observer.OnNewWindowForTab();
+}
+#endif
+
 }  // namespace atom