PluginServiceV2: allow factories to be deprecated

Sometimes it is useful to be able to deprecate components, i.e. to
make sure a particular job does not use a certain set of components
(e.g. a multi-threaded job should not use components that are known to
be not thread-safe).

This can now be achieved by setting the "deprecated" property in the
plugin registry:
```
  Registry::instance().addProperty( "MyTool", "deprecated", "optional message" );
```

In addition, the warning can be turned into an error using the new
`setError` method.