Allow to provide Doctype Configuration from other Plugins

Currently the configuration for templates is provided in

src/main/resources/config/doctypes.properties

This file is located on the classpath an read by a core class.

We have to provided means for other blueprint projects to also register their templates. Maybe we do this via a REST service, an administration page (e.g. for user templates) or a Spring service.

We have to check that the registration information is persistent or recalculated if one of the plugins (core or blueprints) gets reinstalled.