Blender Git Loki

option to disable/enable individual background images

*** use-case:
"I have 10 reference images that overlap each other and every time I want to see one, I need to change the transparency of all the others.
therefore it would be nice to have a little button by each menu to allow enable/disabling individual background images"

To avoid subversioning bump I created a define that is negative (DISABLED) and of course a rna that is a boolean_negative.
Talked with Campbell and he actually prefers this way over do_version, so there it goes.