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/mod.rs

@@ -18,6 +18,7 @@ pub mod idle_notify;
 pub mod image_capture_source;
 pub mod image_copy_capture;
 pub mod input_method;
+pub mod keyboard_layout;
 pub mod keyboard_shortcuts_inhibit;
 pub mod layer_shell;
 pub mod output;