Add to history only when user explicitly searches

This has been bothering me for a while. The current version adds to the find and replace boxes whenever the markers change. You can repro:

* `cmd-f` and type half a word like: `fi` 
* wait half a second
* type the rest of the word like: `find`
* hit up, it's 'fi', wait half a second
* hit up again, it's 'find', wait half a second
* hit up again, it's 'fi', wait half a second
* .... _forever_

In the same case, it adds to the replaceBox's history. If you were to change the replace box's content along with the find box's in the previous example, the replace box's history would be updated. All this without ever hitting enter. No bueno.

This pull only commits to the history when you actually hit enter in one of the boxes, or use find next / prev.