by the MediaPanel as sprites that interact in some standard way with the
mouse.
Modified existing ButtonSprite to work this new way, and it now implements
CommandSprite, ArmingSprite, and DisableableSprite. One method name changed.
git-svn-id: svn+ssh://src.earth.threerings.net/narya/trunk@3362 542714f4-19e9-0310-aa3c-eee0fc999fb1
Ray Greenwell