Empty does not allow Custom Object Properties

[tngreene]

While coming up with a hack for #312, I instructed @airfighter to use an Object with the mesh deleted, then using an Object Property, fake LIGHT_SPILL_CUSTOM, which works, but I'd rather it be me suggesting to use an Empty!

Giving Empties Custom Object Properties would be very easy and I can't see why (other than the idea that we shouldn't have these, which, as the example shows is not good).