AtkValue: refactoring AtkValue

In summary:
  * Stop to use GValue to get/set the value and use doubles instead
  * Include the support for a string description and subranges

https://bugzilla.gnome.org/show_bug.cgi?id=684576
AtkValue: add a testvalue example, with a small AtkValue
 implementation

https://bugzilla.gnome.org/show_bug.cgi?id=684576