Add the ability to delete slider control points using shift+right click

Closes https://github.com/ppy/osu/issues/10672.

In two minds about how this should be implemented but went in this
direction initially. The other way would be to add local handling of
Shift-Right Click inside PathControlPointPiece (which is already doing
mouse handling itself).

osu.Game/Rulesets/Edit/SelectionBlueprint.cs

@@ -143,5 +143,11 @@ namespace osu.Game.Rulesets.Edit
         public virtual Quad SelectionQuad => ScreenSpaceDrawQuad;
 
         public virtual Vector2 GetInstantDelta(Vector2 screenSpacePosition) => Parent.ToLocalSpace(screenSpacePosition) - Position;
+
+        /// <summary>
+        /// Handle to perform a partial deletion when the user requests a quick delete (Shift+Right Click).
+        /// </summary>
+        /// <returns>True if the deletion operation was handled by this blueprint. Returning false will delete the full blueprint.</returns>
+        public virtual bool HandleQuickDeletion() => false;
     }
 }