dbus: Implement org.freedesktop.a11y.Manager protocol

This protocol is now the upstream solution in at-spi2-core/orca for
registering keyboard grabs and watching key events. It should also be a
bit better than the current verious of our custom Wayland protocol for
this purpose.

Like Mutter and Kwin, we currently restrict this to only be called by
the client that holds the name `org.gnome.Orca.KeyboardMonitor` on the
session bus. We also send the `KeyEvent` signal only to registered
watchers, rather than broadcasting, as DBus does by default.

src/shell/mod.rs

@@ -1454,6 +1454,7 @@ impl Common {
         self.popups.cleanup();
         self.toplevel_info_state.refresh(&self.workspace_state);
         self.refresh_idle_inhibit();
+        self.a11y_keyboard_monitor_state.refresh();
     }
 
     pub fn refresh_idle_inhibit(&mut self) {