STYLE: Use a delegate to control extension icon

Change how we manage icons in the extension manager to always set a
'normal' icon on the list item, and to use a delegate to override
whether the item is drawn 'disabled'. This allows us to set an
unchanging icon on the item, and take advantage of the normal drawing
code to draw a disabled icon when appropriate, rather than having to
reset the icon. Right now there is no visible change, but this will
simplify things significantly when we start using 'real' extension icons
rather than the default icon.

git-svn-id: http://svn.slicer.org/Slicer4/trunk@23199 3bd1e089-480b-0410-8dfb-8563597acbee