track recently used/selected languages

[cmyr]

Now that we have a 'languages' menu for selecting the syntax for the current document, there's one obvious improvement we could make: it would be nice if the top had a small 'Recently Used' section, which could show the 4/5 most recent items. This could be based on either what has been sent by "language_changed", or on what has actually been selected by the user. These could be persisted between launches by using NSUserDefaults.