fixed some bugs in gui text box

Source/MonoGame.Extended.Gui/Controls/GuiTextBox.cs

@@ -80,11 +80,17 @@ namespace MonoGame.Extended.Gui.Controls
                     if (SelectionStart < Text.Length)
                         SelectionStart++;
                     break;
+                case Keys.Home:
+                    SelectionStart = 0;
+                    break;
+                case Keys.End:
+                    SelectionStart = Text.Length;
+                    break;
                 default:
                     if (args.Character != null)
                     {
+                        Text = Text.Insert(SelectionStart, args.Character.ToString());
                         SelectionStart++;
-                        Text += args.Character;
                     }
                     break;
             }

Source/MonoGame.Extended.Gui/GuiScreen.cs

@@ -3,7 +3,6 @@ using System.IO;
 using System.Linq;
 using Microsoft.Xna.Framework;
 using Microsoft.Xna.Framework.Content;
-using Microsoft.Xna.Framework.Graphics;
 using MonoGame.Extended.Gui.Controls;
 using MonoGame.Extended.Gui.Serialization;
 using Newtonsoft.Json;

Source/Tests/Sandbox/Game1.cs

@@ -52,7 +52,7 @@ namespace Sandbox
                 {
                     Controls =
                     {
-                        //new GuiLabel { Text = "Hello World" }
+                        new GuiTextBox { Text = "Hello World" }
                     }
                 }
             };