Suggestion: Improvements for RAW demosaic

[Saijin_Naib]

I'd like to see the RAW Demosaic process improved, somewhat along the lines of how DarkTable handles things. 

I like Affinity's ease of use, so setting sensible defaults for Demosaic engine, settings, etc., makes total sense. However, there are instances where a user might want to tweak the Demosaic or use an entirely different algorithm.

For instance, the Markesteijn+fast produces far superior results for X-Trans data to other algorithms in other engines (typically AMaZE-based). This is crucial for IQ, especially when pushing exposure or sharpening at all, basically.