Bug 169852

See issue 65135 for why this is important.

I'm attaching a patch which eliminates all I/O until the submenu is populated, and minimizes it there.  It does the
following things:
 - Use string paths, rather than URLs to store file paths - FileObject -> File -> URL involves multiple blocking I/O
operations
 - Do not check if the files really exist when determining initial enablement of RecentFilesAction
 - Add beep and status message if a file does not exist and cannot be opened

This results in the corner-case situation now possible, where all files in Recent Files have been deleted, but the
action is enabled (its popup contents will be empty).  Given that this is a rare case, and cannot result in the user
trying to actually open something that exists (there will just be no contents in the submenu), this seems worth the
performance benefit.