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_observer.h

@@ -61,6 +61,9 @@ class BrowserObserver {
       bool* prevent_default,
       const std::string& type,
       const base::DictionaryValue& user_info) {}
+
+  // User clicked the native macOS new tab button. (macOS only)
+  virtual void OnNewWindowForTab() {}
 #endif
 
  protected: