Enhancement: Manager - Prevent data loss trough HTML5 localStorage

From the discussion here: https://github.com/modxcms/revolution/issues/3382#issuecomment-43333686

Couldn't leave it and had to try, even with extJS...(god I lost hair...)...and it actually works already pretty well on my test setup...I implemented it that way that the Cancel button forces the localStorage to be cleared.

This very alpha...I definitely need somebody with actual knowledge in ExtJS to have a look at what I did...probably a lot not "how it should be done" things...maybe @rtripault?

There are several things that should be done to make this "real":

1. implement the check if browser supports localStorage and also make sure to catch those users, that manually disable it for whatever reason (this is the tricky part)
2. It doesn't work with TVs...when I change a TV (just tried with an image TV) the onFieldChange method/function sends an "undefined" as object...don't know why...hints on what's wrong btw. how I can get and set the TV field values are highly appreciated
3. It just works for resources...for now, but I think this is where most of unintentional data loss happens...

To see what this actually is supposed to do, a quick screencast:

![modx manager html5localstorage](https://cloud.githubusercontent.com/assets/445501/3001564/010cf378-dd32-11e3-9ee5-e39e43a6a36b.gif)