Interface for inner bundled plugin.
This interface should be used for plugin which are packaged inside others plugins
(in a single JAR plugin file).
Generally you should avoid that as only one plugin can be correctly identified in a JAR
and a descriptor but in some case it can be useful.
You have to implement the getMainPluginClassName() method to return
the main plugin class name so your plugin can be identified.
This class will also hide your plugin from the plugin list in PluginLoader.