Avoid use of the NSString `existsAtPath` category method

We have removed the use of this category method from the rest of the code base. Refactor `FSXcodeProjectDataSource` so that we can delete it entirely.
Remove NSString `isDirectory` category method
Remove NSString `existAtPath` category method

Also rename NSString category from "Path" to "Additions".
Code style fixes
Remove unused includes
Remove unnecessary instance variable

As far as I can tell we only add objects to `_project`, but we never actually use it.