How to implement PQ and HLG curves

The following discussion from !835 (merged) should be addressed:

• @vitalyp started a discussion: (+1 comment)

  this part is in its final form

  Yes, absolutely.

  Thanks for the hints in commit. In order to add PQ and HLG tone curves, an LCMS plug-in is required. I prototyped these on top of the @swick color management branch. Also, Marti agreed that HLG is to be added as part of LCMS, not sure about PQ.

  Question: Do you want to proceed with LCMS changes (create MR into LCMS) or do you prefer a plug-in for new tone curves as we did previously?

  In general, change looks good to me, I need more time to dig more into the details.