Treat tristate as an enum when parsing

This makes it so that you have to explicitly specify
"true" and "false" for the checked state, but it
matches how this enumeration is meant to be used.

gtk/gtkaccessiblevalue.c

@@ -47,6 +47,7 @@
 #include "gtkaccessible.h"
 #include "gtkbuilderprivate.h"
 #include "gtkenums.h"
+#include "gtktypebuiltins.h"
 
 #include <math.h>
 #include <float.h>
@@ -1356,14 +1357,12 @@ gtk_accessible_value_parse (const GtkAccessibleCollect  *cstate,
 
     case GTK_ACCESSIBLE_COLLECT_TRISTATE:
       {
-        gboolean b;
+        int value;
 
         if (collects_undef && strncmp (str, "undefined", 9) == 0)
           res = gtk_undefined_accessible_value_new ();
-        else if (strncmp (str, "mixed", 5) == 0)
-          res = gtk_tristate_accessible_value_new (GTK_ACCESSIBLE_TRISTATE_MIXED);
-        else if (_gtk_builder_boolean_from_string (str, &b, error))
-          res = gtk_boolean_accessible_value_new (b);
+        else if (_gtk_builder_enum_from_string (GTK_TYPE_ACCESSIBLE_TRISTATE, str, &value, error))
+          res = gtk_boolean_accessible_value_new (value);
       }
       break;