Don't apply templated parent if already set.

When applying the `TemplatedParent`, don't move into trees that already have the `TemplatedParent` property set - in this case we can assume that the templated parent has already been set for this tree, and it fixes the problem with menus added in the previous commit.

src/Avalonia.Controls/Primitives/TemplatedControl.cs

@@ -396,7 +396,7 @@ namespace Avalonia.Controls.Primitives
 
             for (var i = 0; i < count; i++)
             {
-                if (children[i] is IStyledElement child)
+                if (children[i] is IStyledElement child && child.TemplatedParent is null)
                 {
                     ApplyTemplatedParent(child, templatedParent);
                 }