Replace Message attached property with gadget types

This is a more efficient way to expose the StandardButton and
StandardIcon enums.

Change-Id: I9d0becf54be2a883d55c9eaeaf10b83b23e59f3c
Reviewed-by: Gabriel de Dietrich <gabriel.dedietrich@digia.com>
Reviewed-by: Alan Alpert <aalpert@blackberry.com>