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

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

src/deref_cell.rs

@@ -0,0 +1,33 @@
+#![allow(dead_code)]
+
+use once_cell::unsync::OnceCell;
+
+/// Wrapper around `OnceCell` implementing `Deref`, and thus also panicking
+/// when not set (or set twice).
+///
+/// To be used in place of `gtk::TemplateChild`, but without xml.
+pub struct DerefCell<T>(OnceCell<T>);
+
+impl<T> DerefCell<T> {
+    #[track_caller]
+    pub fn set(&self, value: T) {
+        if self.0.set(value).is_err() {
+            panic!("Initialized twice");
+        }
+    }
+}
+
+impl<T> Default for DerefCell<T> {
+    fn default() -> Self {
+        Self(OnceCell::default())
+    }
+}
+
+impl<T> std::ops::Deref for DerefCell<T> {
+    type Target = T;
+
+    #[track_caller]
+    fn deref(&self) -> &T {
+        self.0.get().unwrap()
+    }
+}