COMP: Change "EditorExtensionTemplate" layout to support addition of modules/effects.

The re-organization has been done following the approach detailed
in commit r21747.

Also addded the metadata "EXTENSION_ICONURL"

git-svn-id: http://svn.slicer.org/Slicer4/trunk@21750 3bd1e089-480b-0410-8dfb-8563597acbee