Fix for "new" Cycles handling of textures, UI code assumed all texture users use "texture" as prop name, but at least WeightVG modifiers do not.

Fixed by adding a "texture_user_property" member to spacebuts' context, and using it to get the prop identifier in ui script.

Thanks to Brecht for its advice!

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@56362 954f8c5b-7b00-dc11-b283-0030488c597c