WPrefs: Edit proplist menus referenced by WMRootMenu.
Previously, WPrefs could only be used to edit the menu specified in
WMRootMenu.
In a recent commit, the ability to specify a menu in proplist format defined
in another file which is referenced by WMRootMenu was added. However, if a
user attempted to edit such a menu in WPrefs, an error dialog appeared.
We add the ability for WPrefs to read such a menu. After the user makes any
changes, the result is stored in WMRootMenu, and *not* the original file.