Add a warning to enum props' items parameter, to *never* use a generator as value for this param! It works in most cases, but in some it leads to an empty set of values for the enum (see [#33716]), and it's not an easy bug to track down!

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@53485 954f8c5b-7b00-dc11-b283-0030488c597c