Clean up extension points in plugin configuration

Currently all plugins that contain child plugins should explicitly call addExtensionPoint(String name) for each child plugin.
As a convenience Renderservice now checks for the existence of a (multi-value) property ''wicket.extensions' and if it exists it will call addExtensionpoint.

This means that all explicit calls to addExtensionPoint can be removed (and added to the configuration of course)

Also it is important to make it easy to distinguish between extension point configuration properties and 'normal' plugin-specific configuration properties.
This can be done by using a simple naming convention: all extensionpoints start with 'extension.'