Lightmix/Lightselect enhancements

[cgiout]

Hi,
could you consider these improvements, please?
- Lightselect: selecting multiple lights, this generates a lightselect element for each one automatically
- Lightmix: possibility to enable/disable light by checkbox
- Lightmix: group lights per layer (i usually set up scene sets by layers, each containing a set of lights, so it would be useful to group lightselect elements by layer too)
- Lightmix: save lightmix settings

Thank you
Raf