Can't have the xwiki api Context as "context" binding in most of the JSR-223 engines

See XWIKI-3932.

There is a trick in the groovy macro to have access to the Context using "context" binding for now.

The probleme is that in the JSR-223 spec an engine is supposed to provide the ScriptContext as "context" binding so it sounds not possible to have the xwiki Context. Also anyway the Context is an "old" xwiki-core api (like most of what we put in the context) anyway so it will disappear at some point.

• We need to decide it's ok to not have the Context in "context" binding. For example for Velocity and Groovy it's an issue since there is a lot of script based on Context as "context" binding but for any other script we can simply put the Context in "xcontext" or "oldcontext" or any other not "context" binding
• It's also the occasion to of what we will put in the script bindings when we will have the new model and we should prepare that