Add watermark attribute to add watermark to string fields in editor. Have watermark show even when textbox is focused .

Source/Engine/Scripting/Attributes/Editor/WatermarkAttribute.cs

@@ -0,0 +1,43 @@
+using System;
+using FlaxEngine.GUI;
+
+namespace FlaxEngine;
+
+/// <summary>
+/// Used to add a watermark to a string textbox in the editor field
+/// </summary>
+[Serializable]
+[AttributeUsage(AttributeTargets.Field | AttributeTargets.Property)]
+public class WatermarkAttribute : Attribute
+{
+    /// <summary>
+    /// The watermark text.
+    /// </summary>
+    public string WatermarkText;
+
+    /// <summary>
+    /// The watermark color.
+    /// </summary>
+    public uint WatermarkColor;
+
+    /// <summary>
+    /// Initializes a new instance of the  <see cref="WatermarkAttribute"/> class.
+    /// </summary>
+    /// <param name="text">The watermark text.</param>
+    public WatermarkAttribute(string text)
+    {
+        WatermarkText = text;
+        WatermarkColor = 0; // default color of watermark in textbox
+    }
+
+    /// <summary>
+    /// Initializes a new instance of the  <see cref="WatermarkAttribute"/> class.
+    /// </summary>
+    /// <param name="text">The watermark text.</param>
+    /// <param name="color">The watermark color. 0 to use default.</param>
+    public WatermarkAttribute(string text, uint color)
+    {
+        WatermarkText = text;
+        WatermarkColor = color;
+    }
+}

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

@@ -213,7 +213,7 @@ namespace FlaxEngine.GUI
                     color *= 0.6f;
                 Render2D.DrawText(font, _text, color, ref _layout, TextMaterial);
             }
-            else if (!string.IsNullOrEmpty(_watermarkText) && !IsFocused)
+            else if (!string.IsNullOrEmpty(_watermarkText))
             {
                 Render2D.DrawText(font, _watermarkText, WatermarkTextColor, ref _layout, TextMaterial);
             }