UI / World Units:
* Consistency tweak: for properties based on an enum, we hide the buttons and do not grey out. 
Greying out is for properties based on a boolean. 

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@54964 954f8c5b-7b00-dc11-b283-0030488c597c