[Gecode] New variable/view structure in place

[Guido Tack]

Hi everyone.

I updated the sets to reflect a new structure concerning variables and views:

* Variables are the things a user sees, they have a read-only interface, and    
they can be used to post propagators and branchings
* Views are the things that propagators work on.

So far, only the sets have been adjusted to the new schema. The system 
compiles, both with gcc and cl.

Guido

-- 
Guido Tack
http://www.ps.uni-sb.de/~tack