Split out classes and simplify construction of buttons

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

@@ -0,0 +1,66 @@
+// Copyright (c) ppy Pty Ltd <contact@ppy.sh>. Licensed under the MIT Licence.
+// See the LICENCE file in the repository root for full licence text.
+
+using System;
+using osu.Framework.Allocation;
+using osu.Framework.Graphics;
+using osu.Framework.Graphics.Cursor;
+using osu.Framework.Graphics.Sprites;
+using osu.Framework.Input.Events;
+using osuTK;
+using osuTK.Graphics;
+
+namespace osu.Game.Screens.Edit.Compose.Components
+{
+    /// <summary>
+    /// A drag "handle" which shares the visual appearance but behaves more like a clickable button.
+    /// </summary>
+    public sealed class SelectionBoxDragHandleButton : SelectionBoxDragHandle, IHasTooltip
+    {
+        private SpriteIcon icon;
+
+        private readonly IconUsage iconUsage;
+
+        public Action Action;
+
+        public SelectionBoxDragHandleButton(IconUsage iconUsage, string tooltip)
+        {
+            this.iconUsage = iconUsage;
+
+            TooltipText = tooltip;
+
+            Anchor = Anchor.Centre;
+            Origin = Anchor.Centre;
+        }
+
+        [BackgroundDependencyLoader]
+        private void load()
+        {
+            Size *= 2;
+            AddInternal(icon = new SpriteIcon
+            {
+                RelativeSizeAxes = Axes.Both,
+                Size = new Vector2(0.5f),
+                Icon = iconUsage,
+                Anchor = Anchor.Centre,
+                Origin = Anchor.Centre,
+            });
+        }
+
+        protected override bool OnClick(ClickEvent e)
+        {
+            OperationStarted?.Invoke();
+            Action?.Invoke();
+            OperationEnded?.Invoke();
+            return true;
+        }
+
+        protected override void UpdateHoverState()
+        {
+            base.UpdateHoverState();
+            icon.Colour = !HandlingMouse && IsHovered ? Color4.White : Color4.Black;
+        }
+
+        public string TooltipText { get; }
+    }
+}