Update brushes directory structure and file names.
    
    - Previously, brushes were stored directly under a Corporate
      directory. When this branch is merged with tcar branch brushes were
      stored under the same directory which poorly describes its content.
      To be more specific, this update creates the Brushes/Corporate
      directory structure for storing brushes, instead of just Corporate.
    
    - Previously, brushes were using the name symbol for both gbr and xcf.
      In order to be more specific, this update changes the file name of
      brushes from symbol.gbr to centos-symbol-48x48.gbr and from
      symbol.xcf to centos-symbol.xcf.