[plt-scheme] 202.1

The exp-tagged code in CVS for MzScheme and MrEd is now version 202.1.

The main change is a new `message-box/custom' function, which is more
general than `message-box'. For example, you can provide labels for up
to three buttons.

Use this function whenever possible instead of creating your own
message dialog, because `message-box/custom' creates a dialog that
looks [mostly] right for the GUI platform. The framework dialog
function are also ok, of course, because they'll use
`message-box/custom'.

The other 202.1 changes are related:

 - Added 'app, 'caution, and 'stop styles to `message-box'.

 - Added 'app, 'caution, and 'stop icon labels for message%.

 - Mac OS X: file dialogs use the starting directory when one is given,
   and a dialog% with a parent frame is shown as a "sheet".

Matthew