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

[mortend]

Cool :)

Looks good to me!