This patch gives skins developpers the possibility to display the art file
 associated to the current input in an image control.

It features the following :
    - a new 'art' boolean parameter is added for the Image control (skin.dtd)
    - when set to true, the skin engine will reuse this control to display
      the current art file
    - in addition to 'scale' and 'mosaic', the resize parameter can now accept
      a new 'scale2' value, that respects the original aspect ratio of art files