Added an event to subscribe to in TextBoxBase that gets fired when a key is down.

Source/Engine/UI/GUI/Common/TextBoxBase.cs

@@ -129,6 +129,11 @@ namespace FlaxEngine.GUI
         /// </summary>
         public event Action<TextBoxBase> TextBoxEditEnd;
 
+        /// <summary>
+        /// Event fired when a key is down.
+        /// </summary>
+        public event Action<KeyboardKeys> KeyDown;
+
         /// <summary>
         /// Gets or sets a value indicating whether this is a multiline text box control.
         /// </summary>
@@ -1174,6 +1179,7 @@ namespace FlaxEngine.GUI
             var window = Root;
             bool shiftDown = window.GetKey(KeyboardKeys.Shift);
             bool ctrDown = window.GetKey(KeyboardKeys.Control);
+            KeyDown?.Invoke(key);
 
             switch (key)
             {