Add cache to findbar to remember previous search
This merge request is to implement issue #491 (closed). Unfortunately I could not find a way to implement this without the use of the global variable _cache. I looked into making it an instance variable but of course a new FindBar object gets created for each new FileDiff object and obviously a new FileDiff object gets instantiated each time we click a new file in the directory diff page so we would have to go even deeper before we could actually save the value and wed have to pass it all the way back up. This seems very inefficient and the way I implemented it seems to be simple and concise in my opinion.