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/state.rs

@@ -8,6 +8,7 @@ use crate::{
         x11::X11State,
     },
     config::{CompOutputConfig, Config, ScreenFilter},
+    dbus::a11y_keyboard_monitor::A11yKeyboardMonitorState,
     input::{PointerFocusState, gestures::GestureState},
     shell::{CosmicSurface, SeatExt, Shell, grabs::SeatMoveGrabState},
     utils::prelude::OutputExt,
@@ -255,6 +256,7 @@ pub struct Common {
     pub xdg_decoration_state: XdgDecorationState,
     pub overlap_notify_state: OverlapNotifyState,
     pub a11y_state: A11yState,
+    pub a11y_keyboard_monitor_state: A11yKeyboardMonitorState,
 
     // shell-related wayland state
     pub xdg_shell_state: XdgShellState,
@@ -704,6 +706,8 @@ impl State {
 
         let a11y_state = A11yState::new::<State, _>(dh, client_not_sandboxed);
 
+        let a11y_keyboard_monitor_state = A11yKeyboardMonitorState::new(&async_executor);
+
         // TODO: Restrict to only specific client?
         let atspi_state = AtspiState::new::<State, _>(dh, client_has_no_security_context);
 
@@ -764,6 +768,7 @@ impl State {
                 xdg_foreign_state,
                 workspace_state,
                 a11y_state,
+                a11y_keyboard_monitor_state,
                 xwayland_scale: None,
                 xwayland_state: None,
                 xwayland_shell_state,