Implement active/inactive highlight during a Search

[tangledhelix]

As discussed in #561.

Currently when you search (and use next/prev...) the currently focused match is highlighted as you navigate from one match to the next. Other matches, even if they are within the viewport, are treated as normal text and not highlighted.

Leave the currently focused match as-is. Implement a noticeable but de-emphasized highlight on other matches which fall into the viewport. Matches outside the viewport should probably not be considered, to avoid searching the entire file repeatedly. Implement something to run in on_change to reconsider / rehighlight the viewport rather than try to highlight the entire file, which can be too slow.