Add a global variable to hide panel on stella.js

I added a setting by using global variable to hide panel on stella.js.
```js
liberator.globalVariables.stella_hidden_panel = true
```

At first, I thought that all change will be done if I comment in `this.noGUI = true`, but it did not work.
https://github.com/vimpr/vimperator-plugins/blob/master/stella.js#L1876