New functionnality to permit adding files in package
Limiting .css to the only one usable i.e. xx-XX.css
typo
Adding a message if trying to import the xx-XX.css file in the site
folder. + showing the button only when a package has been saved.
Adding an error if a duplicated master core pack (therefore with no lang
set) has not been modified and saved yet.
Merge pull request #207 from infograf768/file_upload

New functionnality to let add files in Core package