Fix undo not available for multiple key types

src/Avalonia.Controls/TextBox.cs

@@ -602,6 +602,7 @@ namespace Avalonia.Controls
                         break;
 
                     case Key.Back:
+                        _undoRedoHelper.Snapshot();
                         if (hasWholeWordModifiers && SelectionStart == SelectionEnd)
                         {
                             SetSelectionForControlBackspace();
@@ -625,11 +626,13 @@ namespace Avalonia.Controls
                             CaretIndex -= removedCharacters;
                             SelectionStart = SelectionEnd = CaretIndex;
                         }
+                        _undoRedoHelper.Snapshot();
 
                         handled = true;
                         break;
 
                     case Key.Delete:
+                        _undoRedoHelper.Snapshot();
                         if (hasWholeWordModifiers && SelectionStart == SelectionEnd)
                         {
                             SetSelectionForControlDelete();
@@ -651,6 +654,7 @@ namespace Avalonia.Controls
                             SetTextInternal(text.Substring(0, caretIndex) +
                                             text.Substring(caretIndex + removedCharacters));
                         }
+                        _undoRedoHelper.Snapshot();
 
                         handled = true;
                         break;
@@ -658,7 +662,9 @@ namespace Avalonia.Controls
                     case Key.Enter:
                         if (AcceptsReturn)
                         {
+                            _undoRedoHelper.Snapshot();
                             HandleTextInput(NewLine);
+                            _undoRedoHelper.Snapshot();
                             handled = true;
                         }
 
@@ -667,7 +673,9 @@ namespace Avalonia.Controls
                     case Key.Tab:
                         if (AcceptsTab)
                         {
+                            _undoRedoHelper.Snapshot();
                             HandleTextInput("\t");
+                            _undoRedoHelper.Snapshot();
                             handled = true;
                         }
                         else