Add hooks for custom window decorations

This is a first use of the new hooks system, which allows customizing
cosmic-comp at compile-time.
In this case, the view() function of CosmicWindow / CosmicStack is
hooked and the hook can change what is rendered as the header bar.

Signed-off-by: Yureka <yuka@yuka.dev>

src/hooks.rs

@@ -0,0 +1,47 @@
+// SPDX-License-Identifier: GPL-3.0-only
+
+use crate::shell::element::stack::{
+    CosmicStackInternal, DefaultDecorations as DefaultStackDecorations, Message as StackMessage,
+};
+use crate::shell::element::window::{
+    CosmicWindowInternal, DefaultDecorations as DefaultWindowDecorations, Message as WindowMessage,
+};
+use std::sync::{Arc, OnceLock};
+
+/// An _unstable_ interface to customize cosmic-comp at compile-time by providing
+/// hooks to be run in specific code paths.
+#[derive(Default, Debug, Clone)]
+pub struct Hooks {
+    pub window_decorations:
+        Option<Arc<dyn Decorations<CosmicWindowInternal, WindowMessage> + Send + Sync>>,
+    pub stack_decorations:
+        Option<Arc<dyn Decorations<CosmicStackInternal, StackMessage> + Send + Sync>>,
+}
+
+pub static HOOKS: OnceLock<Hooks> = OnceLock::new();
+
+pub trait Decorations<Internal, Message>: std::fmt::Debug {
+    fn view(&self, state: &Internal) -> cosmic::Element<'_, Message>;
+}
+
+impl Decorations<CosmicWindowInternal, WindowMessage>
+    for Option<Arc<dyn Decorations<CosmicWindowInternal, WindowMessage> + Send + Sync>>
+{
+    fn view(&self, window: &CosmicWindowInternal) -> cosmic::Element<'_, WindowMessage> {
+        match self {
+            None => DefaultWindowDecorations.view(window),
+            Some(deco) => deco.view(window),
+        }
+    }
+}
+
+impl Decorations<CosmicStackInternal, StackMessage>
+    for Option<Arc<dyn Decorations<CosmicStackInternal, StackMessage> + Send + Sync>>
+{
+    fn view(&self, window: &CosmicStackInternal) -> cosmic::Element<'_, StackMessage> {
+        match self {
+            None => DefaultStackDecorations.view(window),
+            Some(deco) => deco.view(window),
+        }
+    }
+}