RelativePanel does not measure Correctly

[danwalmsley]

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>
```

[NaBian]

As shown, I think the UWP rendering is wrong(the left image is UWP, which has a gray background on the right)

<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"
               RelativePanel.RightOf="RedRect" />

    <Rectangle x:Name="GreenRect" Fill="Green" 
               Height="44"
               RelativePanel.Below="RedRect" 
               RelativePanel.AlignLeftWith="RedRect" 
               RelativePanel.AlignRightWith="BlueRect"/>
</RelativePanel>

[danwalmsley]

Awesome, I agree UWP is wrong, and the behavior is now as I expect. Thankyou for your hard work!