Add new interface to make splash plugin go to idle

Add new interface to make splash plugin go to idle

There are times when we want to make the splash
screen stop any pending animations and got to an
idle state.  For instance, right before resuming,
or right before loading up GDM.  It may take a
few frames for the splash screen to get there though.
This interface tells the splash screen it needs to
idle and provides a trigger for it to fire when it's
actually made it to an idle state.