add Snapping to the ScrollView

[ichan-mb]

Scroll snapping allows you to lock the viewport to a certain location after a user has finished scrolling. You can use the SnapAlignment property to control the lock position. Three lock positions are available: ' Start Center, and End.

The setup that works now is with a StackPanel (Horizontal or Vertical)

<ScrollView LayoutMode="PreserveVisual" Height="400">
    <StackPanel>
        <Each Count="100" Reuse="Frame" >
            <Panel Color="#AAA">
                <Text Value="Data {= index() }"/>
            </Panel>
        </Each>
    </StackPanel>
    <ScrollViewSnap SnapAlignment="Center" />
</ScrollView>

It's required to use LayoutMode="PreserveVisual" on the ScrollView and the child element in the StackPanel has an equal size

This PR contains:

• [ ] Changelog
• [ ] Documentation
• [ ] Tests