Fix IsScrollInertiaEnabled warning.

Fix the annoying warning:

```
Error in binding to 'Avalonia.Input.GestureRecognizers.ScrollGestureRecognizer'.'IsScrollInertiaEnabled': 'Null value in expression '{empty}' at '$self.TemplatedParent'
```

This is a bit of a hack; we take advantage of the fact that `IsScrollInertiaEnabled` is an attached property and set it on the `ScrollContentPresenter` where the other bound properties are located.

Fixes #11148

src/Avalonia.Themes.Fluent/Controls/ScrollViewer.xaml

@@ -33,11 +33,12 @@
                                   VerticalSnapPointsType="{TemplateBinding VerticalSnapPointsType}"
                                   HorizontalSnapPointsAlignment="{TemplateBinding HorizontalSnapPointsAlignment}"
                                   VerticalSnapPointsAlignment="{TemplateBinding VerticalSnapPointsAlignment}"
-                                  Padding="{TemplateBinding Padding}">
+                                  Padding="{TemplateBinding Padding}"
+                                  ScrollViewer.IsScrollInertiaEnabled="{TemplateBinding IsScrollInertiaEnabled}">
             <ScrollContentPresenter.GestureRecognizers>
               <ScrollGestureRecognizer CanHorizontallyScroll="{Binding CanHorizontallyScroll, ElementName=PART_ContentPresenter}"
                                        CanVerticallyScroll="{Binding CanVerticallyScroll, ElementName=PART_ContentPresenter}"
-                                       IsScrollInertiaEnabled="{Binding IsScrollInertiaEnabled, RelativeSource={RelativeSource TemplatedParent}}" />
+                                       IsScrollInertiaEnabled="{Binding (ScrollViewer.IsScrollInertiaEnabled), ElementName=PART_ContentPresenter}"/>
             </ScrollContentPresenter.GestureRecognizers>
           </ScrollContentPresenter>
           <ScrollBar Name="PART_HorizontalScrollBar"

src/Avalonia.Themes.Simple/Controls/ScrollViewer.xaml

@@ -14,11 +14,12 @@
                                   VerticalSnapPointsType="{TemplateBinding VerticalSnapPointsType}"
                                   HorizontalSnapPointsAlignment="{TemplateBinding HorizontalSnapPointsAlignment}"
                                   VerticalSnapPointsAlignment="{TemplateBinding VerticalSnapPointsAlignment}"
-                                  Background="{TemplateBinding Background}">
+                                  Background="{TemplateBinding Background}"
+                                  ScrollViewer.IsScrollInertiaEnabled="{TemplateBinding IsScrollInertiaEnabled}">
             <ScrollContentPresenter.GestureRecognizers>
               <ScrollGestureRecognizer CanHorizontallyScroll="{Binding CanHorizontallyScroll, ElementName=PART_ContentPresenter}"
                                        CanVerticallyScroll="{Binding CanVerticallyScroll, ElementName=PART_ContentPresenter}"
-                                       IsScrollInertiaEnabled="{Binding IsScrollInertiaEnabled, RelativeSource={RelativeSource TemplatedParent}}" />
+                                       IsScrollInertiaEnabled="{Binding (ScrollViewer.IsScrollInertiaEnabled), ElementName=PART_ContentPresenter}"/>
             </ScrollContentPresenter.GestureRecognizers>
           </ScrollContentPresenter>
           <ScrollBar Name="PART_HorizontalScrollBar"