Set size of a group.

When set the elements will be arranged in as many groups of size GroupSize as will fit on the display.

Note that GroupSize and GroupCount are exclusive, and should not be set at the same time.

<CollectionPanel Color="Black" GroupSize="12">
    <Each Count="150">
        <Circle Margin="1" Width="10" Height="10" Color="Yellow" />
        <Rectangle Margin="1" Width="10" Height="40" Color="Red" />
        <Rectangle Margin="1" CornerRadius="4" Width="10" Height="20"  Color="Teal" />
    </Each>
</CollectionPanel>