Override this class to provide additional nodes in 'Available Elements' tree on 'Output Layout' tab of an artifact editor. This tree
contains elements which are usually included into an artifact so don't include optional and rarely used items there. All packaging elements
may be added by clicking on '+' icon on the toolbar above 'Output Layout' tree anyway.
The implementation should be registered in plugin.xml file:
<extensions defaultExtensionNs="com.intellij">
<packaging.sourceItemProvider implementation="qualified-class-name"/>
</extensions>