accessible: Add a way to hide accessibles

Similar to gtk_widget_should_layout(), add a
gtk_accessible_should_present() function that backends can
use to determine whether an accessible should be presented
or not.

Ways to make a widget not presented in a11y:
- hide the widget
- set its role to NONE
- make it have a NULL AT context

We will use this in future to hide the GtkText inside
an entry, since the Text implementation will be done
by the wrapper.

gtk/gtkaccessible.c

@@ -47,6 +47,7 @@
 #include "gtkatcontextprivate.h"
 #include "gtkenums.h"
 #include "gtktypebuiltins.h"
+#include "gtkwidget.h"
 
 #include <glib/gi18n-lib.h>
 
@@ -649,3 +650,15 @@ gtk_accessible_platform_changed (GtkAccessible               *self,
   gtk_at_context_update (context);
 }
 
+gboolean
+gtk_accessible_should_present (GtkAccessible *self)
+{
+  if (GTK_IS_WIDGET (self) &&
+      !gtk_widget_get_visible (GTK_WIDGET (self)))
+    return FALSE;
+
+  if (gtk_accessible_get_accessible_role (self) == GTK_ACCESSIBLE_ROLE_NONE)
+    return FALSE;
+
+  return TRUE;
+}

gtk/gtkaccessibleprivate.h

@@ -40,4 +40,6 @@ const char *    gtk_accessible_role_to_name     (GtkAccessibleRole  role,
 void            gtk_accessible_platform_changed (GtkAccessible               *self,
                                                  GtkAccessiblePlatformChange  change);
 
+gboolean        gtk_accessible_should_present   (GtkAccessible     *self);
+
 G_END_DECLS