AMIGAOS: Make use of the new drawer icon

On AmigaOS the user can customize every icon including drawers.

This is a customized drawer icon for ScummVM with the original "S"
scaled on top of it.

This icon may change with further revisions of AmigaOS4 (if the
default icon images change).
Merge pull request #501 from raziel-/patch-3

AMIGAOS: Add and make use of a new drawer icon