Opens a multitasking message box with the
<txt> Text to use as the message
<title> Title of the window
<spr> Lower sprite to use, if cannot be
found then "error" is used.
<dflt> Text of default button (RETURN/F5)
<b> Text of next button (ESCAPE/F4)
<c> Text of third button (F3)
<d> Text of final button (F2)
If a button's text is "" then that button is
hidden. <dflt> being selected will return 1,
<b> will return 2 and so on.
The sprite can come from the Wimp sprite pool
or the "Sprites" file. The upper sprite will
be the application sprite or "switcher" if
that cannot be found.
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