Limiting extension package choice to non-core files

Partly solving https://github.com/joomla-projects/com_localise/issues/126 as no chosen.
When creating a new Extension package, the choice will not include core files.
We will get for example:
![screen shot 2014-06-30 at 17 08 56](https://cloud.githubusercontent.com/assets/869724/3431208/87b0c89c-0068-11e4-9d5e-bd0ee23933e0.png)