revamp per-module histogram method - mainly affecting modules tonecurve and levels

* histogram data can now be collected by all modules by setting module->request_histogram
* multi-instance modules will get histograms individually generated for the different instances
* histograms for all data channels L, a, b are generated and displayed in tonecurve module
* modules acting in RGB color space will get histograms with channels R, B, G and Grey
Merge branch 'histogram'