Fix caret not displaying when focused via Tab key.

Also set caret position to end when selecting all, as textboxes when selected should scroll to display the end of the text.

src/Avalonia.Controls/TextBox.cs

@@ -347,7 +347,7 @@ namespace Avalonia.Controls
 
             if (IsFocused)
             {
-                DecideCaretVisibility();
+                _presenter?.ShowCaret();
             }
         }
 
@@ -364,14 +364,7 @@ namespace Avalonia.Controls
             {
                 SelectAll();
             }
-            else
-            {
-                DecideCaretVisibility();
-            }
-        }
 
-        private void DecideCaretVisibility()
-        {
             _presenter?.ShowCaret();
         }
 
@@ -975,6 +968,7 @@ namespace Avalonia.Controls
         {
             SelectionStart = 0;
             SelectionEnd = Text?.Length ?? 0;
+            CaretIndex = SelectionEnd;
         }
 
         private bool DeleteSelection()