bug 1066830 hide theme menu if there is no theme app.

For the better future of Settings App, we will move all these `try_*` scripts into `root/panels/xxx_item.js` and share the `apps` variable instead of calling the API twice (or even more). 

So please follow https://github.com/mozilla-b2g/gaia/blob/v2.1/apps/settings/js/panels/root/panel.js and make the same  file for this.