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/main.rs

@@ -1,7 +1,7 @@
 // SPDX-License-Identifier: GPL-3.0-only
 
 fn main() {
-    if let Err(err) = cosmic_comp::run() {
+    if let Err(err) = cosmic_comp::run(Default::default()) {
         tracing::error!("Error occured in main(): {}", err);
         std::process::exit(1);
     }