Sorry for bringing this up late, but I think it'd actually be better to do the filtering more centrally for consistency -- for example, Find in Files should also ignore binary files.

So, let's move this to FileIndexManager, right after the shouldShow() check in _addFileToIndexes().  That will filter binary files out of all search-related functionality while leaving them visible in the project tree.  Exactly what we want, I think.

(As I mentioned before, [once images are openable](https://trello.com/c/l9AcILkC/preview-images) we'll need to adjust this to distinguish readable binary files from non-readable, but we can deal with that later)