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.

gtk/gtkthemingengine.c

@@ -321,7 +321,7 @@ gtk_theming_engine_is_state_set (GtkThemingEngine *engine,
   g_return_val_if_fail (GTK_IS_THEMING_ENGINE (engine), 0);
 
   priv = engine->priv;
-  return gtk_style_context_is_state_set (priv->context, state);
+  return gtk_style_context_is_state_set (priv->context, state, NULL);
 }
 
 G_CONST_RETURN GtkWidgetPath *