RelativePanel Below and RightOf Arranges correctly, but LeftOf and Above doestn #437

Describe the bug
A clear and concise description of what the bug is.

<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.LeftOf="rect1" />
</RelativePanel>

Expected:

Actual:

same happens the the Yellow is above rect1.

Can be worked around but likely an easy fix.

To Reproduce
Steps to reproduce the behavior:

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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