Editor search and replace

Here it is: basic 'search' and 'search and replace' for the editor.

I'll be honest and straight up about this: these functions need a lot of love. Namely, they are both 'one-off' only, meaning that finding the next occurance for searching or replacing means you have to run the command all over again.

They also are quite similar to each other; they share some cleanup and setup code that there's no obvious way (to me at least) to refactor into a shared function.

However, given how long this functionality has been on the wishlist, I figured that something is better than nothing. And with the core bread and butter functionality done, maybe someone else can figure out how to make it nicer. Perhaps it could be left the way it is to encourage the player to work for their tools. ;)

Given the less-polished nature of this particular patchset, I wouldn't be hurt to hear it needs more work.

As always, if you'd like anything else to go with this patchset, let me know.