Have icon_for_app_id run in a background thread

This should probably be faster, but it's good for things like this to
not block the UI thread regardless.

We could probably also cache for multiple apps with the same ID. Not
sure if there's a good way to detect changes to the icon for an app id
(Not really needed, probably?)

src/desktop_info.rs

@@ -5,13 +5,17 @@ use freedesktop_desktop_entry::DesktopEntry;
 use itertools::Itertools;
 use std::path::PathBuf;
 
-pub fn icon_for_app_id(app_id: String) -> Option<PathBuf> {
-    Some(
-        desktop_info_for_app_ids(vec![app_id])
-            .into_iter()
-            .next()?
-            .icon,
-    )
+pub async fn icon_for_app_id(app_id: String) -> Option<PathBuf> {
+    tokio::task::spawn_blocking(|| {
+        Some(
+            desktop_info_for_app_ids(vec![app_id])
+                .into_iter()
+                .next()?
+                .icon,
+        )
+    })
+    .await
+    .unwrap()
 }
 
 #[allow(dead_code)]