incorrect addon extension point declaration

in http://download.eclipse.org/gemoc/docs/nightly/_contributing.html#_developing_new_addons
, it explains that we can click on Class*: in the extension editor to create the initial version of a class implementing IEngineAddon

The procedure is correct but the generated file imports `org.eclipse.gemoc.gemoc_language_workbench.api.engine_addon.IEngineAddon` instead of `org.eclipse.gemoc.xdsmlframework.api.engine_addon.IEngineAddon`

The fix is relatively simple: remove the wrong import and use Eclipse quickfix to add the correct import statement.

The exsd file declaring the extension point must probably be reviewed.

(note, if looking into his exsd, we should also check to other GEMOC extension points ...,
for example also adding some descriptions as it is displayed by the plugin editor wizard)