Clear TemplatedParent of ex-template children.

This prevents them still being bound to their ex-TemplatedParent. Fixes
#435.

src/Perspex.Controls/Primitives/TemplatedControl.cs

@@ -251,7 +251,15 @@ namespace Perspex.Controls.Primitives
         {
             if (!_templateApplied)
             {
-                VisualChildren.Clear();
+                if (VisualChildren.Count > 0)
+                {
+                    foreach (var child in this.GetTemplateChildren())
+                    {
+                        child.SetValue(TemplatedParentProperty, null);
+                    }
+
+                    VisualChildren.Clear();
+                }
 
                 if (Template != null)
                 {
@@ -318,6 +326,11 @@ namespace Perspex.Controls.Primitives
         /// <param name="e">The event args.</param>
         protected virtual void OnTemplateChanged(PerspexPropertyChangedEventArgs e)
         {
+            if (_templateApplied && VisualChildren.Count > 0)
+            {
+                _templateApplied = false;
+            }
+
             _templateApplied = false;
             InvalidateMeasure();
         }

tests/Perspex.Controls.UnitTests/Primitives/TemplatedControlTests.cs

@@ -311,6 +311,39 @@ namespace Perspex.Controls.UnitTests.Primitives
             Assert.True(raised);
         }
 
+        [Fact]
+        public void Applying_New_Template_Clears_TemplatedParent_Of_Old_Template_Children()
+        {
+            var target = new TestTemplatedControl
+            {
+                Template = new FuncControlTemplate(_ => new Decorator
+                {
+                    Child = new Border(),
+                })
+            };
+
+            target.ApplyTemplate();
+
+            var decorator = (Decorator)target.GetVisualChildren().Single();
+            var border = (Border)decorator.Child;
+
+            Assert.Equal(target, decorator.TemplatedParent);
+            Assert.Equal(target, border.TemplatedParent);
+
+            target.Template = new FuncControlTemplate(_ => new Canvas());
+
+            // Templated children should not be removed here: the control may be re-added
+            // somewhere with the same template, so they could still be of use.
+            Assert.Same(decorator, target.GetVisualChildren().Single());
+            Assert.Equal(target, decorator.TemplatedParent);
+            Assert.Equal(target, border.TemplatedParent);
+
+            target.ApplyTemplate();
+
+            Assert.Null(decorator.TemplatedParent);
+            Assert.Null(border.TemplatedParent);
+        }
+
         private static IControl ScrollingContentControlTemplate(ContentControl control)
         {
             return new Border