[UpDown control] Forward HintAssist.HintPaddingBrush into nested TextBox of UpDown

[nicolaihenriksen]

Ran into a small issue when using the UpDown variants inside of a custom background (e.g. a ColorZone). For the outlined style, the hint sits on top of the outline border when it is floating, and has a background set to hide the outline below the hint.

For non-default backgrounds, the HintAssist.HintPaddingBrush can be used to ensure this behavior is maintained. However, this AP was not forwarded through in the UpDown case. This PR does just that. Also fixed indentation, so possibly hiding whitespaces in the diff will show you a better diff.

For this XAML

<materialDesign:ColorZone x:Name="MyColorZone" Mode="PrimaryLight" Padding="10" Width="150">
  <materialDesign:DecimalUpDown Style="{StaticResource MaterialDesignOutlinedDecimalUpDown}"
                                materialDesign:HintAssist.HintPaddingBrush="{Binding ElementName=MyColorZone, Path=Background}"
                                materialDesign:HintAssist.Hint="The hint" />
</materialDesign:ColorZone>

Before (AP not forwarded)

After (AP forwarded)