refactor: overhaul .desktop and image resolver systems

Rewrites the desktop file parser code and image resolver code to introduce caching system and make fully async. They should be much faster now.

BREAKING CHANGE: The `icon_theme` setting has been moved from per-bar to top-level

src/channels.rs

@@ -125,6 +125,12 @@ where
     fn recv_glib<F>(self, f: F)
     where
         F: FnMut(T) + 'static;
+
+    /// Like [`BroadcastReceiverExt::recv_glib`], but the closure must return a [`Future`].
+    fn recv_glib_async<Fn, F>(self, f: Fn)
+    where
+        Fn: FnMut(T) -> F + 'static,
+        F: Future;
 }
 
 impl<T> BroadcastReceiverExt<T> for broadcast::Receiver<T>
@@ -152,4 +158,29 @@ where
             }
         });
     }
+
+    fn recv_glib_async<Fn, F>(mut self, mut f: Fn)
+    where
+        Fn: FnMut(T) -> F + 'static,
+        F: Future,
+    {
+        glib::spawn_future_local(async move {
+            loop {
+                match self.recv().await {
+                    Ok(val) => {
+                        f(val).await;
+                    }
+                    Err(broadcast::error::RecvError::Lagged(count)) => {
+                        tracing::warn!(
+                            "Channel lagged behind by {count}, this may result in unexpected or broken behaviour"
+                        );
+                    }
+                    Err(err) => {
+                        tracing::error!("{err:?}");
+                        break;
+                    }
+                }
+            }
+        });
+    }
 }