Make Meshlights recognition more reliable

Until now, the "Select Meshlights" operator and Meshlights stats on info header were using their own individual approach, and both were missing the emission nodes that could be inside a group (they were counting the stand alone Emission nodes, not grouped).

So now I made it take into account emission nodes that could live inside a group, and put it all into one single function to re-use in both cases.

For Emission nodes inside a group, it checks that both the Emission node *and* the group node itself is connected. Why counting a group that is connected if the Emission node is not connected to anything?

Much more reliable now, I'd say 100%! :)