*Allowing the creation of File packages + other improvements

The component needs to be able to create Core lang packs or extensions file lang packages.
See model here: http://docs.joomla.org/J2.5:Making_non-core_language_packs
We now can make both:
![screen shot 2014-06-15 at 09 19 31](https://cloud.githubusercontent.com/assets/869724/3280384/7addc7bc-f45d-11e3-9e17-ab7cc42d46c8.png)

For the moment it only does for one language at a time.
To test, install a default 3.3.1 and install a core lang pack
Open the Translations view and filter by Origin: Core to see the state of the files and populate the database
Go to Packages and choose new Core pack
choose all files that should go in (for example do not add com_localise files)
for the version enter 3.3.1.1, the language concerned and save, Without quitting this edit page click on the download button.

Install a third party component (for example patchtester)
Translate in the Translations view for the same language you installed previously
Go to packages
Click New File Package and choose the 2 patch tester files

Click on download

The PR also corrects/improve other aspects, like filtering by Core files in translations view 
(@Todo: 
Add xx-XX.finder_cli.ini which does not follow "normal" file names.
Allow creation/editing of the xx-XX.localise.php, by adding it to the view and letting copy from reference in a codemirror editable instance.
Allow creation/editing of the xx-XX.css, by adding it to the view in a codemirror editable instance.
Allow, when making a core pack, adding some stuff in the media folder (calendar for Farsi is a good exemple)

All code style is not perfect and I have left a lot of older code in.
This should be considered as a step in the refactoring process.

According to our coding standards we don't use underscore for defining a private class property or method

```php
// Please in future pulls fix this:
protected function _allowAdd

// for:
protected function allowAdd
```