Fix dark interface lock icon not visible (used in lock ratio preferences) by adding a light outline
    
    * The chain icon is not a rectangle (128x256)
    * The icon is now loaded thrue the pixbuf of an iconset and scladed down  (resized) manually
    
    Fix issue reported in forum V1.0 RC#1:
    http://www.synfig.org/forums/viewtopic.php?f=1&t=8107&start=30#p28378