add type coercion helper to ListItem objects

src/album.rs

@@ -150,6 +150,10 @@ impl ListItem for Album {
         }
     }
 
+    fn as_listitem(&self) -> Box<ListItem> {
+        Box::new(self.clone())
+    }
+
     fn display_left(&self) -> String {
         format!("{}", self)
     }

src/artist.rs

@@ -139,6 +139,10 @@ impl ListItem for Artist {
         }
     }
 
+    fn as_listitem(&self) -> Box<ListItem> {
+        Box::new(self.clone())
+    }
+
     fn display_left(&self) -> String {
         format!("{}", self)
     }

src/playlist.rs

@@ -107,6 +107,10 @@ impl ListItem for Playlist {
         }
     }
 
+    fn as_listitem(&self) -> Box<ListItem> {
+        Box::new(self.clone())
+    }
+
     fn display_left(&self) -> String {
         self.name.clone()
     }

src/track.rs

@@ -151,6 +151,10 @@ impl ListItem for Track {
         current.map(|t| t.id == self.id).unwrap_or(false)
     }
 
+    fn as_listitem(&self) -> Box<ListItem> {
+        Box::new(self.clone())
+    }
+
     fn display_left(&self) -> String {
         format!("{}", self)
     }

src/traits.rs

@@ -28,6 +28,8 @@ pub trait ListItem: Sync + Send + 'static {
     fn artist(&self) -> Option<Artist> {
         None
     }
+
+    fn as_listitem(&self) -> Box<ListItem>;
 }
 
 pub trait ViewExt: View {