changelayer - triggered after a layer name change, order change, opacity
change, params change, visibility change (actual visibility, not the layer's
visibility property) or attribution change (due to extent change). Listeners
will receive an event object with *layer* and *property* properties. The
*layer* property will be a reference to the changed layer. The *property*
property will be a key to the changed property (name, order, opacity, params,
visibility or attribution).
Listening to visibility changes can be done at the layer level with a