Make osc skinnable

Make osc skinnable. put graphics in USERNAME/OSC_Skin-folder to skin
elements. Its the same concept as osu it have. all skinnable files are
named in Lib/System/Skinnables.rb.

You can put osu skin files in this folder, osc is loading this files as
default for each new skin and copy it automatically to the new skin
directory. This files could be changed using osc normally.