a11y: provide a way to ensure clutter accessibility

If gtk_init is called after clutter_init, it can override clutter
AtkUtil implementation. In that situation, we can't say that
the accessibility is enabled, as the root object would be wrong.

In order to provide a way to prevent this:

* clutter_get_accessibility_enabled returns true of false
  depending on the current AtkUtil implemented
* cally_accessibility_init always override AtkUtil implementation.