Enhance the categories screen

[amwebexpert]

Once this pull request is merged, enhance the categories screen:

• enhance the TextService by adding a Future<Map<ApiCategory, File>> getCategoryCacheFilesMap() async
  • NOTE: for performance purpose, we could have null as File value, so we know the file does not exists instead of leaving the consumer call file.existsSync on each file later...
  • could also be implemented Future<Map<ApiCategory, CacheModel>> getCategoryCacheFilesMap() async to avoid implementation leak!
  • see Directory('my-dir').listSync to list all files of a directory
• add an File param to the CategoryWidget
• add detail to the CategoryWidget:
  • timestamp of the last cached content (if the file exists)
  • category language label
• bonus: leading icon for the language
• trailing icon to show if any cached content exists