Show Extensions Folder support for Path Finder

The previous code was opening Finder specifically with some
AppleScript.  This commit uses the 'standard' API
NSWorkspace selectFile:inFileViewerRootedAtPath:
to do this.  This allows Finder overrides like Path Finder
to be launched when the user selects the menu item
Help / Show Extensions Folder

see: https://github.com/adobe/brackets/issues/2899