a new hook to manage client class 'class_cmds'

For yang/netconf and config backend we need a way to manage client-classes so a 'class_cmds' which provides the usual function: adding, querying (aka get) and deleting client classes from the running configuration.

Open questions:

• where to put it? a priori support package

• what to do with dependent expressions? If a class is just removed this can lead to a configuration which cannot be reloaded so a priori expressions of following classes should be checked against dangling references.

• do we need a change/update? as the class order is critical IMHO we should provide one.

Note that class-list should simply return the list of configured class names. Add should append the new class to the end.