Added missing source file for placeholder tool strip text box

Source/Core/BuilderMono.csproj

@@ -184,6 +184,9 @@
       <DependentUpon>ExternalCommandControl.cs</DependentUpon>
     </Compile>
     <Compile Include="Controls\FolderSelectDialog.cs" />
+    <Compile Include="Controls\PlaceholderToolStripTextBox.cs">
+      <SubType>Component</SubType>
+    </Compile>
     <Compile Include="Controls\Scripting\TextEditorControl.cs" />
     <Compile Include="Controls\TransparentTrackBar.cs">
       <SubType>Component</SubType>

Source/Core/Controls/PlaceholderToolStripTextBox.cs

@@ -0,0 +1,77 @@
+#region ================== Namespaces
+
+using System;
+using System.Drawing;
+using System.Runtime.InteropServices;
+using System.Windows.Forms;
+
+#endregion
+
+namespace CodeImp.DoomBuilder.Controls
+{
+	// This is based on https://stackoverflow.com/questions/50918225/how-to-add-placeholder-text-to-toolstriptextbox
+	[ToolboxBitmap(typeof(ToolStripTextBox))]
+	public class PlaceholderToolStripTextBox : ToolStripTextBox
+	{
+		#region ================== DLL Imports
+
+		[DllImport("user32.dll", CharSet = CharSet.Auto)]
+		private static extern int SendMessage(IntPtr hWnd, int msg, int wParam, string lParam);
+
+		#endregion
+
+		#region ================== Constants
+
+		private const int EM_SETCUEBANNER = 0x1501;
+
+		#endregion
+
+		#region ================== Variables
+
+		private string placeholder;
+
+		#endregion
+
+		#region ================== Properties
+
+		public string PlaceholderText
+		{
+			get { return placeholder; }
+			set
+			{
+				placeholder = value;
+				UpdatePlaceholderText();
+			}
+		}
+
+		#endregion
+
+		#region ================== Constructors
+
+		public PlaceholderToolStripTextBox()
+		{
+			Control.HandleCreated += Control_HandleCreated;
+		}
+
+		#endregion
+
+		#region ================== Events
+
+		private void Control_HandleCreated(object sender, EventArgs e)
+		{
+			if (!string.IsNullOrEmpty(placeholder))
+				UpdatePlaceholderText();
+		}
+
+		#endregion
+
+		#region ================== Methods
+
+		private void UpdatePlaceholderText()
+		{
+			SendMessage(Control.Handle, EM_SETCUEBANNER, 0, placeholder);
+		}
+
+		#endregion
+	}
+}