gtk: Rework AccessibleExtManual

Adds compile-time type safety to accessible properties, relations and
states.

examples/custom_widget/ex_button/imp.rs

@@ -31,6 +31,9 @@ impl ObjectSubclass for ExButton {
 
         // Make it look like a GTK button.
         klass.set_css_name("button");
+
+        // Make it appear as a button to accessibility tools.
+        klass.set_accessible_role(gtk::AccessibleRole::Button);
     }
 }
 
@@ -39,13 +42,17 @@ impl ObjectImpl for ExButton {
         self.parent_constructed(obj);
 
         // Create the child label.
-        let child = gtk::Label::new(Some("Hello world!"));
+        let label = "Hello world!";
+        let child = gtk::Label::new(Some(label));
         child.set_parent(obj);
         *self.child.borrow_mut() = Some(child.upcast::<gtk::Widget>());
 
         // Make it look like a GTK button with a label (as opposed to an icon).
         obj.add_css_class("text-button");
 
+        // Tell accessibility tools the button has a label.
+        obj.update_property(&[gtk::accessible::Property::Label(label)]);
+
         // Connect a gesture to handle clicks.
         let gesture = gtk::GestureClick::new();
         gesture.connect_released(|gesture, _, _, _| {

examples/custom_widget/ex_button/mod.rs

@@ -4,7 +4,8 @@ use gtk::glib;
 
 glib::wrapper! {
     pub struct ExButton(ObjectSubclass<imp::ExButton>)
-        @extends gtk::Widget;
+        @extends gtk::Widget,
+        @implements gtk::Accessible;
 }
 
 impl Default for ExButton {