[development] Move multiple theme per site support to contrib module

[Moshe Weitzman]

> but having the notion of "enabled" and "default" theme is essential to 
> us, we use groups on our sites and having the ability for each group to 
> choose their own theme seems a kicker, people feel that's a space belong 
> to themselves.  If there are  other ways to achieve the same result it 
> will be fine, but we don't want to lose the  user specified theme 
> capabilities  -- i.e. make sure the contribute module for multi-theme 
> support is available at the same time when it disappears  from the core.

if you are referring to organic groups module, then it will surely continue 
to offer theme choice among the non enabled themes. no worries.