Allow all files to be opened by drag and drop

@peterflynn @RaymondLim 

Remove `FileUtils.isTextFile()` since it was giving too many false negatives. The drag and drop code now accepts *all* files, and relies on FILE_OPEN to determine which files can or cannot be opened.

This required adding an optional `silent` parameter to `FILE_OPEN`. When true, errors are not shown when opening files.

See [this comment](https://github.com/adobe/brackets/pull/4406#discussion_r5125307) for more details.