Add GtkAccessibleValue

Add GtkAccessibleValue

All accessible properties and states may have one of the following
types:

 - true/false
 - true/false/undefined
 - true/false/mixed/undefined
 - reference (to another UI element)
 - reference list
 - integer
 - number (real numerical value)
 - string
 - token (one of a limited set of allowed values)
 - token list

See: https://www.w3.org/WAI/PF/aria/states_and_properties#propcharacteristic_value

The GtkAccessibleValue is a simple reference counted type that can be
"subclassed" to implement each value type.

This initial commit adds GtkAccessibleValue and the basic subclasses for
plain boolean, tristate (true/false/undefined), and token types,
including statically allocated values that can be shared instead of
allocated.