Make interaction "active" an ol.Object property

This PR makes `ol.interaction.Interaction` inherit from `ol.Object` and makes `'active'` an `ol.Object` property. With this `change:active` and `propertychange` events are emitted when `setActive` is called on an interaction.
We could also make it possible to pass arbitrary properties to interactions. So users could name their interactions for example. But I guess this can be done with a separate PR.