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
@peterflynn Did you see [this comment](https://github.com/adobe/brackets/issues/9444#issuecomment-58968093) where user is complaining about Brackets "freezing" with a 3MB file on a system with a (spinning) hard drive?