RelativePanel does not measure Correctly #794
Description
Describe the bug
A clear and concise description of what the bug is.
Top is UWP, bottom is WPF handycontrol version,
Since the recent fix we do not return the measured size or minimum size the content takes up.
This is because we are returning the maximum available size from MeasureOverride, regardless of Horizontal and VerticalAlignment.
To Reproduce
```
<handy:RelativePanel Background="Gray" HorizontalAlignment="Left" VerticalAlignment="Top">
<Rectangle x:Name="RedRect" Fill="Red" Height="44" Width="44"/>
<Rectangle x:Name="BlueRect" Fill="Blue"
Height="44" Width="88"
handy:RelativePanel.RightOf="RedRect" />
<Rectangle x:Name="GreenRect" Fill="Green"
Height="44"
handy:RelativePanel.Below="RedRect"
handy:RelativePanel.AlignLeftWith="RedRect"
handy:RelativePanel.AlignRightWith="BlueRect"/>
<Rectangle Fill="Orange"
handy:RelativePanel.Below="GreenRect"
handy:RelativePanel.AlignLeftWith="BlueRect"
handy:RelativePanel.AlignRightWithPanel="True"
handy:RelativePanel.AlignBottomWithPanel="True"/>
</handy:RelativePanel>
```