Put an upper limit on UTF8 file sizes

This is for #7223

* Start the discussion at limiting files to 4MB and under before we refuse to load them
* Display a message if we've reached the limit opening a file
* Implemented in AppShellFIleSystem which is really the only place this could go without rearchitecting a lot of file system work but looking for feedback there too
@JeffryBooher This may be a reasonable default to help people with slow systems from opening files that will freeze Brackets, but what about putting this in a preference for people with better hardware?

In [this comment](https://github.com/adobe/brackets/issues/9444#issuecomment-58971328), a 3MB file that was sent in as causing slowdowns works fine on my system. I'm sure it has a lot to do with hardware specs, so it would be nice if I was not limited to opening 4MB files.