#19 DocSkeleton quit dialog needs Don't Save button

[Chris Waco]

When you run DocSkeleton, modify a document, and then
attempt to quit, the dialog that comes up has "Save" and
"Cancel" buttons, but no "Don't Save" button.

This makes it impossible to quit without saving changes.

Note that clicking in the close box produces a proper save
dialog.

Also, the dialog should be a sheet in MacOS X.