Fix for "DatePicker with Floating Hint Style's Top Margin/Padding: Issue # 1966

[HClausing]

Matching from Trigger Property="wpf:HintAssist.IsFloating" Value="True" from MaterialDesignTheme.TextBox.xaml :

                    <Trigger Property="wpf:HintAssist.IsFloating" Value="True">
                        <Setter TargetName="border" Property="Margin" Value="0 18 0 0" />
                        <Setter TargetName="HintBackgroundBorder" Property="Background" Value="{Binding Path=(wpf:HintAssist.Background), RelativeSource={RelativeSource TemplatedParent}}" />
                    </Trigger>

[Keboo]

Adding issue link #1966