a11y: Compute the label for a GtkATContext

a11y: Compute the label for a GtkATContext

The ARIA spec defines the mechanism for determining the name of an
accessible element—see §4.3 of the WAI-ARIA spec.

We follow the specification as much as it makes sense for GTK to do
so:

 1. if the element is hidden, return an empty string
 1. if the element has a labelled-by relation set, retrieve the
    label of the related element
 2. if the element has a label property set, use the value of
    the property
 3. if neither labelled-by nor label attributes are set, we use
    the role to compute the name:
   - for a `range` role, we return the contents of the value of
     the `value-text` or `value-now` properties
   - for any other role, we return a textual representation of
     the GtkAccessibleRole enumeration value