[Specs] ScrollAware revealer (Flutter, Facebook, toolbar and bottom bar hide on scroll)

[roubachof]

Would be great to have a simple away to implement the most used "hide on scroll" UX pattern.
2 simple attached properties should be enough:

Name	Type	Description
ScrollViewer	ScrollViewer	the ScrollViewer controlling the reveal effect
RevealedPosition	Enumeration	Enum: Left, Top, Bottom, Right

 <Grid>

    <ScrollViewer x:Name="ScrollViewer">
        <StackPanel>
            <Item />
            <Item />
            <Item />
			...
        </StackPanel>
    </ScrollViewer>

	<StackPanel Padding="20"
	            VerticalAlignment="Top"
	            local:ScrollAware.RevealedPosition="Top"
	            local:ScrollAware.ScrollViewer="{x:Bind ScrollViewer}"
	            Background="#333333">
	    ...
	</StackPanel>

	<StackPanel Width="100"
	            Height="300"
	            HorizontalAlignment="Left"
	            VerticalAlignment="Center"
	            local:ScrollAware.RevealedPosition="Left"
	            local:ScrollAware.ScrollViewer="{x:Bind ScrollViewer}"
	            Background="#333333"
	            CornerRadius="0,30,30,0">
	    ...
	</StackPanel>

	<StackPanel Width="100"
	            Height="300"
	            HorizontalAlignment="Right"
	            VerticalAlignment="Center"
	            local:ScrollAware.RevealedPosition="Right"
	            local:ScrollAware.ScrollViewer="{x:Bind ScrollViewer}"
	            Background="#333333"
	            CornerRadius="30,0,0,30">
	    ...
	</StackPanel>
	
	<StackPanel Padding="20"
	            VerticalAlignment="Bottom"
	            local:ScrollAware.RevealedPosition="Bottom"
	            local:ScrollAware.ScrollViewer="{x:Bind ScrollViewer}"
	            Background="#333333">
	    ...
	</StackPanel>

</Grid>

reveal_windows.mp4

In the current implementation there is an abstract ScrollAwareController that is used to subscribe to scroll events and provides the need properties such as velocity, offset delta and vertical offset.

This could be used to implement #768

I already implemented a working component here: https://github.com/sharpnado-pro/Uno.PoC
@kazo0 and @pictos are admin, you can request them the read permission.
You call also see the whole scroll aware code here: https://gist.github.com/roubachof/d77099ededf9e75d47453996a3250c75

ScrollAwareReavealAndroid.mp4

[Xiaoy312]

we could maybe add a property to adjust the triggering scroll direction? which should be defaulted to scroll up