Centralise cases of performing actions on the current selection

By moving this to a central location, we can avoid invoking the
EditorChangeHandler when there is no selection made. This helps
alleviate the issue pointed out in
https://github.com/ppy/osu/issues/11901, but not fix it completely.

osu.Game/Screens/Edit/Compose/Components/BlueprintContainer.cs

@@ -495,8 +495,7 @@ namespace osu.Game.Screens.Edit.Compose.Components
                 // Apply the start time at the newly snapped-to position
                 double offset = result.Time.Value - movementBlueprints.First().HitObject.StartTime;
 
-                foreach (HitObject obj in Beatmap.SelectedHitObjects)
-                    obj.StartTime += offset;
+                Beatmap.PerformOnSelection(obj => obj.StartTime += offset);
             }
 
             return true;