Introduce a service through which which cms wicket models can be invalidated

CMS wicket models at this moment are invalidated if for example it is a JcrNodeModel and the backing jcr node gets modified. Whether this jcr node gets modified by the current cms visitor or via someone else, every editor in the cms that has a JcrNodeModel for that node, gets its model invalidated.

Now, there can be use cases where the model needs to be reloaded while there is no actual jcr node change. For example when the workflow integrates with an external BPM engine : If the change triggered by a workflow step should alter the available workflow menu items, but the workflow step only triggers a change in the bpm system, the menu won't reload.

Hence, we need a service to which we can post node events (that will trigger a wicket model reload in the cms)