Fix Issue #7846 - "Renaming a file to the same name but a different case produces an error"

Fixes issues:
[#7846](https://github.com/adobe/brackets/issues/7846) - "Renaming a file to the same name but a different case produces an error"
[#8127](https://github.com/adobe/brackets/issues/8127) - "Error Renaming File"

Workaround issue with [NSFileManager -moveItemAtPath:toPath:error:] that occurs renaming with a filename change that differs in case-only (eg `test.js` and `TEST.js`).  This is supported by the Mac OSX filesystem, so it should be allowed.  To workaround this shortcoming, this change uses a temporary, intermediate filename to do the rename in two steps.
I think these two operations should either have proper error handling if something goes wrong during the rename or we should use the `replaceItemAtURL` method of `NSFileManager` to ensure that this is an atomic operation.

https://developer.apple.com/library/mac/documentation/Cocoa/Reference/Foundation/Classes/NSFileManager_Class/Reference/Reference.html#//apple_ref/occ/instm/NSFileManager/replaceItemAtURL:withItemAtURL:backupItemName:options:resultingItemURL:error: