Update graphics/viewnior to 1.6.

Changes:
 * Viewnior 1.6
  * New features:
   * Added support for the back & forward buttons of your mouse
   * Updated translations
  * Fixes:
   * Mouse scroll behaviour and actions settings
 * Viewnior 1.5
  * New features:
   * Disable Wallpaper support during compilation
   * Updated translations
  * Fixes:
   * Preferences menu position
   * Nitrogen wallpaper support
 * Viewnior 1.4
  * New features:
   * Read EXIF and IPTC metadata
   * Nitrogen support
   * Updated translations
  * Fixes:
   * Preserve metadata after image modification
   * Rendidering artifacts when panning
