[Proposal] Move XWiki.getService()/getPortalService() to an Exo plugin

[Vincent Massol]

Hi,

I'd like to move the public APIs getService() and getPortalService()  
in a separate Exo plugin. Advantages:

1) less cruft in XWiki classes
2) removes one dependency on Exo in XWiki core
3) better separation of concern

ok with everyone?

Thanks
-Vincent