Play nice with https://github.com/shpakovski/Popup.

Many projects use shpakovski/Popup, but AXStatusItemPopup has a problem when a Popup-based application is also in the menu bar:

1. Click the Popup application menu bar icon.
2. Click the AXStatusItemPopup menu bar icon.
3. See below result.

![screenshot_problem](https://f.cloud.github.com/assets/2056763/1710823/9cd6b1e4-613c-11e3-868a-e54fc0af9739.png)

This commit fixes the problem by presenting a dummy (empty) NSMenu from the NSStatusItem, which causes the Popup panel to close when the AXStatusItemPopup menu bar icon is clicked.