Add "last played" sort mode to song select

Note that this will consider the most recent play of any beatmap in
beatmap set groups for now, similar to other sort methods.

osu.Game/Screens/Select/Carousel/CarouselBeatmapSet.cs

@@ -81,6 +81,9 @@ namespace osu.Game.Screens.Select.Carousel
                 case SortMode.DateAdded:
                     return otherSet.BeatmapSet.DateAdded.CompareTo(BeatmapSet.DateAdded);
 
+                case SortMode.LastPlayed:
+                    return -compareUsingAggregateMax(otherSet, b => (b.LastPlayed ?? DateTimeOffset.MinValue).ToUnixTimeSeconds());
+
                 case SortMode.BPM:
                     return compareUsingAggregateMax(otherSet, b => b.BPM);