Stop using a blend of translucent match-highlight + modified selection
color to indicate the current Find/Replace match. Use a simpler system
where we use two different, opaque highlight styles for general matches and
the current match, moving the current-match highlight marker around the
editor as the user navigates between matches.

This makes it easier to create themes; and gives much more flexibility in
choosing the two highlight colors (some color combinations were previously
impossible).
Merge pull request #8603 from adobe/pflynn/simpler-search-highlight

Improve search highlight maintainability / themability