Fix for r57316: SPACE_EMPTY *is* needed in RNA enum for now, exported keymaps abuse it...

So for now, static enum is back to its previous form, and the menu one is dynamically generated by a callback. Not nice, but works.

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@57339 954f8c5b-7b00-dc11-b283-0030488c597c