perf: use unstable sorting when async

Unstable sorting should be slightly faster than stable sorting, and if
the vector was built asynchronously then preserving the initial order
doesn't matter too much.

Continue to use stable sorting where the vector is not built
asynchronously, or if the vector is partially sorted (e.g. when new
elements are pushed to a sorted vector).

cosmic-applet-audio/src/mpris_subscription.rs

@@ -171,7 +171,7 @@ impl State {
         filter_firefox_players(&mut players);
 
         // pre-sort by path so that the same player is always selected
-        players.sort_by(|a, b| a.name().cmp(b.name()));
+        players.sort_unstable_by(|a, b| a.name().cmp(b.name()));
 
         let mut state = Self {
             conn,