Implementing package duplication

See https://github.com/joomla-projects/com_localise/issues/165

This will let use master packages to create a specific language package where the files to include are already highlighted.
![screen shot 2014-07-05 at 11 34 52](https://cloud.githubusercontent.com/assets/869724/3486217/b66aed6c-0427-11e4-8eab-b86d8bab4d96.png)