Add Wayland layer-shell wrapper, behind layer-shell feature

Requires patched GTK4 from
https://github.com/pop-os/gtk4/tree/layer-shell_impish.

src/lib.rs

@@ -1 +1,6 @@
+mod deref_cell;
+#[cfg(feature = "layer-shell")]
+pub mod wayland;
+#[cfg(feature = "layer-shell")]
+mod wayland_custom_surface;
 pub mod x;