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/mac/atom_application_delegate.mm

@@ -81,4 +81,8 @@ continueUserActivity:(NSUserActivity*)userActivity
   return browser->ContinueUserActivity(activity_type, *user_info) ? YES : NO;
 }
 
+- (IBAction)newWindowForTab:(id)sender {
+  atom::Browser::Get()->NewWindowForTab();
+}
+
 @end