RelativePanel does not measure correct size in certain configuration #431

Describe the bug
A clear and concise description of what the bug is.
Expected behavior (according to UWP) is that relativepanel sizes to the content, unless alignment = stretch.

<RelativePanel Background="White" VerticalAlignment="Top" HorizontalAlignment="Left">
            <Rectangle Name="rect1" Height="20" Width="20" Fill="Red"  />
            <Rectangle Name="rect2" Height="10" Width="10" Fill="Yellow" RelativePanel.AlignBottomWithPanel="True" />
          </RelativePanel>

in uwp this produces..

but in HandyControl.RelativePanel we get...

The issue is that Measure does not create a graph and calculate the correct size.

So arrange tells the graph is has the available space.

To Reproduce
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1. Go to '...'
2. Click on '....'
3. Scroll down to '....'
4. See error

Expected behavior
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Screenshots
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Environment (please complete the following information):

• .net: [e.g. 4.6.2]
• IDE [e.g. vs2017]
• Version [e.g. 1.0.0]

Additional context
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