Add an override/exception pref for dark mode and RFP

This MR would add a patch adding a new setting (privacy.override_rfp_for_color_scheme) which would re-enable websites being informed of a user's dark/light theme preference with RFP remaining enabled.

Considering that seemed to be an issue that was often solved by just disabling RFP completely, this would probably be a solution with much less privacy impact.

It's set to false by default. It would probably be good to add this as a lockPref to the librewolf.cfg, as to ensure it's only deliberately changed and not "by accident" in about:config, for example.

Should resolve one or two open issues here and there, linux#170 (closed) and windows#27 (closed), for example.