Optimize timeline visibility

Due to "Recursion detected" bugs in slint, we need to calculate the
layout of the timeline in Rust. This is ugly, but works.

src/api.rs

@@ -162,6 +162,10 @@ impl Message<GetAssetThumbnail> for Api {
         msg: GetAssetThumbnail,
         _ctx: &mut Context<Self, Self::Reply>,
     ) -> Self::Reply {
+        if let Some(thumbnail) = self.thumbnails.get(&msg.id).cloned() {
+            return Ok(thumbnail);
+        }
+
         let response = self
             .client
             .assets()
@@ -182,9 +186,13 @@ impl Message<GetAssetThumbnail> for Api {
             thumbnail.height(),
         );
 
-        Ok(Arc::new(AssetThumbnail {
+        let thumbnail = Arc::new(AssetThumbnail {
             id: msg.id,
             thumbnail: pixel_buffer,
-        }))
+        });
+
+        self.thumbnails.insert(msg.id, Arc::clone(&thumbnail));
+
+        Ok(thumbnail)
     }
 }