GtkStyleContext: Add animation machinery.

GtkStyleContext: Add animation machinery.

Animation regions are confined through gtk_style_context_[push|pop]_animatable_region,
anything that's painted between these calls will be invalidated if an animation starts
for any of the regions in the stack.

gtk_style_context_notify_state_change() may be called from widgets to indicate a
change for a given GtkStateType, in that case an animation will be started if there
is an animation description for the widget/state.