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.
Good catch @ingorichter.  I've added error handling to catch the intermediate rename failure.  It's still a two-step process though.  Even with the above function `replaceItemAtURL`, we'd still have to make two calls since that function attempts to replace contents in the same filename.  We actually want to rename, so that's why we use the intermediate filename.