Refactores the default OpenTox services, to have them as plugins

This allows the adding of, for example, a local OpenTox service to be configured by a plugin. That means that users do not have to go through preferences pages, etc, but can simply install a plugin.

As a demo, a localhost plugin is provided, in addition to the earlier Ambit and Muenchen services. The latter no longer exists and the matching plugin has been removed from the feature.