Fix [#33825] Menu list widget does not work on UV and Vertex Colors

Own stupid mistake in recent UI list refactor - those two lists are a good example where a non-void 'id' is necessary, as they use the same class in the same window... Else, the same object is shared by the two, which can't work! :)

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@53718 954f8c5b-7b00-dc11-b283-0030488c597c