Reworked client setup functionality

Right now the client register functionality works with entrypoints and some hacks in the setup form for the examplecontent profile.

To enable a bigger policy package which includes multiple client configurations in separate and conditionally subpackages, i reworked the register and setup functionality:

* Replaced the old client and policy registration with two new zcml directives `registerPolicy` and `registerClient`.
* Implements an `ClientConfigurationRegistry` Utility, which handles and store all the registered clients.
* Reworked the setup form, which use now the `ClientConfigurationRegistry` utility.
* Rebuild registration of the `examplecontent` clients, use the new zcml-directives.


@jone @lukasgraf could you take a look?