Fix Issue #8127- "Error Renaming File"

Fix [Issue #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.