Read & output combo indices in timestamps in catch editor

Addresses https://github.com/ppy/osu/discussions/33912 for catch
specifically. Code copy-pasted from osu! ruleset.

I'm leaving taiko be because new combo still doesn't make any sense in
taiko. It'd probably either have to be object index in beatmap period
instead of index in combo, or the `kdkdkdkdkkkdkdkkkdkkdkd` notation
people use.

osu.Game.Rulesets.Catch/Edit/CatchHitObjectComposer.cs

@@ -3,6 +3,7 @@
 
 using System.Collections.Generic;
 using System.Linq;
+using System.Text.RegularExpressions;
 using osu.Framework.Allocation;
 using osu.Framework.Bindables;
 using osu.Framework.Graphics;
@@ -219,5 +220,40 @@ namespace osu.Game.Rulesets.Catch.Edit
             distanceSnapGrid.StartTime = sourceHitObject.GetEndTime();
             distanceSnapGrid.StartX = sourceHitObject.EffectiveX;
         }
+
+        #region Clipboard handling
+
+        public override string ConvertSelectionToString()
+            => string.Join(',', EditorBeatmap.SelectedHitObjects.Cast<CatchHitObject>().OrderBy(h => h.StartTime).Select(h => (h.IndexInCurrentCombo + 1).ToString()));
+
+        // 1,2,3,4 ...
+        private static readonly Regex selection_regex = new Regex(@"^\d+(,\d+)*$", RegexOptions.Compiled);
+
+        public override void SelectFromTimestamp(double timestamp, string objectDescription)
+        {
+            if (!selection_regex.IsMatch(objectDescription))
+                return;
+
+            List<CatchHitObject> remainingHitObjects = EditorBeatmap.HitObjects.Cast<CatchHitObject>().Where(h => h.StartTime >= timestamp).ToList();
+            string[] splitDescription = objectDescription.Split(',');
+
+            for (int i = 0; i < splitDescription.Length; i++)
+            {
+                if (!int.TryParse(splitDescription[i], out int combo) || combo < 1)
+                    continue;
+
+                CatchHitObject? current = remainingHitObjects.FirstOrDefault(h => h.IndexInCurrentCombo + 1 == combo);
+
+                if (current == null)
+                    continue;
+
+                EditorBeatmap.SelectedHitObjects.Add(current);
+
+                if (i < splitDescription.Length - 1)
+                    remainingHitObjects = remainingHitObjects.Where(h => h != current && h.StartTime >= current.StartTime).ToList();
+            }
+        }
+
+        #endregion
     }
 }