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: System menu navigation in a menuless MDI window leads to a
 crash

A top-level-window<%>'s on-move method is not reliably called; also,
 the result from get-x, get-y, and is-iconized? are not reliable