inspector: Show a11y information

inspector: Show a11y information

Show a tab for  accessibility information.

This shows the role and the accessible attributes
(states, properties, relations).

For now, changing the values is not possible, and
we only show the explicitly set values. In the future,
we want to show the attributes that are relevant for
the role, regardless of whether they are set or not,
and allow changing some of the attributes (the ones
that are not fully managed by GTK itself).