skins2: implement art display in image controls

skins2: implement art display in image controls

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