revealer: Only clip child when animating

revealer: Only clip child when animating

Currently, GtkRevealer clips the child if the transition type is
sliding, regardless of whether the animation had already ended. An
example where that is a problem would be in Nautilus: the file
operations popover button is animated on reveal to draw attention, but,
given that the button is in turn stashed inside a revealer with a
sliding animation, things suddenly fall apart.