Add incsearch setting to toggle find highlighting before finishing query

[ericdwang]

Defaults to true to match current behavior.

Unfortunately in Firefox it's sometimes possible to trigger a mapping during a find query by pausing and then typing again (e.g. pressing "r" after pausing reloads the page). I noticed there's code specific to Firefox (visualUpdatedForFirefox) to steal focus back from the page to the find bar, but it seems there's a small window between the blur and focus where keypresses aren't sent to the find bar.

This just allows avoiding the problem (not sure what the actual fix would be), but I think it'd be useful anyway as some people might prefer non-incremental search.