Improve context menus to make the UX/UI more consistent (#923)

* Add save option to context menu of all possible ListItems

* Add play options to context menus

* Fix for playlists and tracks

* Move playback controls into main menu

src/traits.rs

@@ -47,6 +47,17 @@ pub trait ListItem: Sync + Send + 'static {
         None
     }
 
+    #[allow(unused_variables)]
+    #[inline]
+    fn is_saved(&self, library: Arc<Library>) -> Option<bool> {
+        None
+    }
+
+    #[inline]
+    fn is_playable(&self) -> bool {
+        false
+    }
+
     fn as_listitem(&self) -> Box<dyn ListItem>;
 }