Implicitly make cluster variables available to plugins

At the moment, plugin configurations need each of their properties to be explicitly configured. This leads to a lot of configuration overhead, such as
<sv:property sv:name="wicket.id" sv:type="String">
<sv:value>${wicket.id}</sv:value>
</sv:property>
for a number of properties. (in document editor definitions, these include 'engine', 'mode', 'validator.id', 'wicket.id' and 'wicket.model' for each field plugin)