Improve search highlight maintainability / themability

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).