If no plugins are configured to be started in the config file, apps
start up, register no plugins, and do nothing. This is almost
guaranteed to be wrong behavior in all situations, so it should be
easy to specify some plugins that should always be started. This
commit makes this so.

By listing headers among the sources, we don't confuse cmake at all
since it knows better than to compile those, but we do make life easier
on IDEs with CMake support, since the IDE will now pick up the headers
as more source code for the project.