Add FocusedTextBox abstraction.

osu.Game/Graphics/UserInterface/FocusedTextBox.cs

@@ -0,0 +1,49 @@
+using OpenTK.Graphics;
+using OpenTK.Input;
+using osu.Framework.Input;
+using System;
+using System.Linq;
+
+namespace osu.Game.Graphics.UserInterface
+{
+    public class FocusedTextBox : OsuTextBox
+    {
+        protected override Color4 BackgroundUnfocused => new Color4(10, 10, 10, 255);
+        protected override Color4 BackgroundFocused => new Color4(10, 10, 10, 255);
+
+        public Action Exit;
+
+        private bool focus;
+        public bool HoldFocus
+        {
+            get { return focus; }
+            set
+            {
+                focus = value;
+                if (!focus)
+                    TriggerFocusLost();
+            }
+        }
+
+        protected override bool OnFocus(InputState state)
+        {
+            var result = base.OnFocus(state);
+            BorderThickness = 0;
+            return result;
+        }
+
+        protected override void OnFocusLost(InputState state)
+        {
+            if (state.Keyboard.Keys.Any(key => key == Key.Escape))
+            {
+                if (Text.Length > 0)
+                    Text = string.Empty;
+                else
+                    Exit?.Invoke();
+            }
+            base.OnFocusLost(state);
+        }
+
+        public override bool RequestingFocus => HoldFocus;
+    }
+}