Automatic download of language files

[mvlcek]

To simplify the language configuration for new users, I suggest to:

• improve the Extend API with a function to return all language files, and
  
• put all CKEditor language files into a publicly accessible area on get-simple.info
  

in order to implement a GetSimple core functionality to select and download languages from the admin settings:

• new sidebar entry in Settings: Add language
  
• which shows a list of languages from Extend, appropriate for the installed version of GetSimple
  
• user can select one language and press a button Download
  
• language ZIP is downloaded, unzipped and put into /admin/lang/ (overwriting older files)
  
• based on the name of the file put into /admin/lang/ the language file of CKEditor - if it exists - is also downloaded and put into the appropriate directory.
  

This way it doesn't matter that much if only a view languages are distributed with GetSimple and the user can always easily get the newest language files.