Blender Git Loki

Texture painting:

Do not generate materials/images/UVs if they are missing.

Now we spawn a panel ("Missing Data") with operators to generate the missing data and
pop a warning if user tries to paint without them.

The reason we have reverted this is that it is too easy to end up with more textures
than we wanted. It was impossible to enter texture paint without having textures added,
and code makes too many assumptions about what user may want.

Discussed during Sunday's meeting.

This might be a candidate for 2.72a but I'm not sure how other artists will take this
(and how refined and crash-free it is), better make a few iterations first.

And for interested parties...test please, don't wait until after a release to poke with such issues.

Also, add slot operator now adds a new unconnected image node in cycles. Only
used in the "Missing Data" panel. This should be a separate commit but I am squashing it into the same commit because
it relies too much on changes done here and can be reverted easily if complainstorm occurs again.