Refactor app key color generation to be faster
See the commit messages.
This goes most of the way to fixing #1148 (closed), but doesn’t yet do anything about contrast in featured app tiles. I’ll do that in a follow-up MR. This now also adds an algorithm to choose the most appropriate key colour as a background colour for
GsFeatureTile, fixing #1148 (closed) completely.
Fixes: #1148 (closed)