Add PaneTitle to NavigationViewTemplateSettings

active/NavigationView/NavigationViewTemplateSettings/PaneTitle.md

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+# Background
+Xaml controls are typically implemented using an element tree that’s defined in a template.
+For example the template for a ComboBox control has a TextBlock in it to display the selected value.
+Several controls have properties that are only expected to be useful to these templates, 
+and so there’s a common pattern for the control to expose them in a TemplateSettings property. 
+For example [ComboBox.TemplateSettings](https://docs.microsoft.com/en-us/uwp/api/Windows.UI.Xaml.Controls.ComboBox.TemplateSettings) 
+is used by the ComboBox’s template. See [this doc page](https://docs.microsoft.com/en-us/windows/uwp/xaml-platform/template-settings-classes) for more details. 
+
+The new API here adds a PaneTitle property to NavigationView's existing
+[TemplateSettings property](https://docs.microsoft.com/uwp/api/Windows.UI.Xaml.Controls.NavigationViewTemplateSettings).
+This helps the control template decide if it should show the
+[NavigationView.PaneTitle](https://docs.microsoft.com/uwp/api/Windows.UI.Xaml.Controls.NavigationView.PaneTitle)
+property, as it should be ignored if the
+[NavigationView.PaneHeader](https://docs.microsoft.com/uwp/api/Windows.UI.Xaml.Controls.NavigationView.PaneHeader)
+property is set. (The RS5 PaneHeader property supersedes/replaces the RS4 PaneTitle property.)
+
+
+# Description
+Provides calculated PaneTitle that can be referenced as TemplatedParent sources when defining templates for a NavigationView control. Not intended for general use.
+
+# Examples
+
+```
+[webhosthidden]
+unsealed runtimeclass NavigationViewTemplateSettings : Windows.UI.Xaml.DependencyObject
+{
+    ...
+
+    {
+        String PaneTitle{ get; };
+        static Windows.UI.Xaml.DependencyProperty PaneTitleProperty{ get; };
+    }
+}
+```
+
+# Remarks
+This is intended for use in the style of the `NavigationView` control. Sample usage:
+
+```xaml
+    <Style TargetType="local:NavigationView">
+ ...
+                                    <TextBlock
+                                        x:Name="PaneTitleTextBlock" 
+                                        Grid.Column="0"
+                                        Text="{Binding RelativeSource={RelativeSource TemplatedParent}, Path=TemplateSettings.PaneTitle}"
+                                        HorizontalAlignment="Left"
+                                        VerticalAlignment="Center"
+                                        Style="{StaticResource NavigationViewItemHeaderTextStyle}"/>
+```
+
+# API Notes
+
+## Class: NavigationViewTemplateSettings 
+| Member Name | Description |
+|:- |:--|
+| PaneTitle | The NavigationView.PaneTitle displayed in the control. Empty when NavigationView.PaneHeader is set |
+