Add cosmic-keyboard-layout-unstable-v1 protocol

https://github.com/pop-os/cosmic-protocols/pull/28

Allows clients like `cosmic-applet-input-sources` to show which layout
group is active and update it. Currently this is implemented by update
the `xkb_config` to change the layout order.

src/wayland/handlers/keyboard_layout.rs

@@ -0,0 +1,10 @@
+// SPDX-License-Identifier: GPL-3.0-only
+
+use crate::state::State;
+use crate::wayland::protocols::keyboard_layout::{KeyboardLayoutHandler, KeyboardLayoutState};
+
+impl KeyboardLayoutHandler for State {
+    fn keyboard_layout_state(&mut self) -> &mut KeyboardLayoutState {
+        &mut self.common.keyboard_layout_state
+    }
+}