Add temperature/tint sliders

[inukshuk]

Closes #463

[luzpaz]

Any progress on this ?

[inukshuk]

We made good progress with the shader initially but hit an impasse: the balancing works well for one channel (e.g. blue) but we need to find a different approach that works on all channels equally (we'd like this to be a single slider not three). We need to study/research into color theory a bit more to progress ourselves. Also happy to accept help on this of course. Basically, we have this set up but we need a shader that works with a single parameter (or parameters which we can map to a slider or similar widget).