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.h

@@ -183,6 +183,11 @@ class Browser : public WindowListObserver {
   // Tell the application to open a url.
   void OpenURL(const std::string& url);
 
+#if defined(OS_MACOSX)
+  // Tell the application to create a new window for a tab.
+  void NewWindowForTab();
+#endif  // defined(OS_MACOSX)
+
   // Tell the application that application is activated with visible/invisible
   // windows.
   void Activate(bool has_visible_windows);