Windows and Mac OS: "primitive" dialogs don't get closed by
 custodian-shutdown-all

Mac OS: std file dialog and print dialogs block MzScheme threads

Windows 95/98/Me: System menu navigation in a menuless MDI window
 leads to a crash.

The result from get-x, get-y, and is-iconized? of
 top-level-window<%> are not completely reliable.