message: Create checkbox-specific validation error template for proper red border display (win)
diff: xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
  xmlns:mah="http://metro.mahapps.com/winfx/xaml/controls"
  xmlns:togglDesktop="clr-namespace:TogglDesktop">
  + <ResourceDictionary.MergedDictionaries>
  + <ResourceDictionary Source="ErrorTemplate.xaml" />
  + </ResourceDictionary.MergedDictionaries>
  +
  + <ControlTemplate x:Key="CheckBoxValidationErrorTemplate">
  + <AdornedElementPlaceholder x:Name="placeholder">
  + <Grid SnapsToDevicePixels="True">
  + <Rectangle x:Name="PopupTargetElement"
  + Margin="-2 0"
  + DataContext="{Binding ElementName=placeholder}"
  + StrokeThickness="0" />
  + <Border x:Name="ValidationErrorElement"
  + DataContext="{Binding ElementName=placeholder}">
  + </Border>
  +
  + <mah:CustomValidationPopup x:Name="ValidationPopup"
  + Placement="Top"
  + AllowsTransparency="True"
  + IsOpen="False"
  + PlacementTarget="{Binding ElementName=PopupTargetElement, Mode=OneWay}"
  + Child="{StaticResource ValidationPopupRoot}">
  + </mah:CustomValidationPopup>
  + </Grid>
  + </AdornedElementPlaceholder>
  +
  + <ControlTemplate.Triggers>
  + <MultiDataTrigger>
  + <MultiDataTrigger.Conditions>
  + <Condition Binding="{Binding ElementName=placeholder, Path=AdornedElement.IsKeyboardFocusWithin, Mode=OneWay}" Value="True" />
  + <Condition Binding="{Binding ElementName=placeholder, Path=AdornedElement.(Validation.HasError), Mode=OneWay}" Value="True" />
  + </MultiDataTrigger.Conditions>
  + <Setter TargetName="ValidationPopup" Property="IsOpen" Value="True" />
  + </MultiDataTrigger>
  + </ControlTemplate.Triggers>
  + </ControlTemplate>
  +
  <!-- ********************************** CheckBoxStyle ********************************** -->
  <!-- adapted from http://brianlagunas.com/free-metro-light-and-dark-themes-for-wpf-and-silverlight-microsoft-controls/ -->
  <Style x:Key="MahApps.Styles.CheckBox.Win10" TargetType="CheckBox">
  <Setter Property="BorderBrush" Value="{DynamicResource Toggl.SelectionElements.Border}" />
  <Setter Property="BorderThickness" Value="1" />
  + <Setter Property="Validation.ErrorTemplate" Value="{DynamicResource CheckBoxValidationErrorTemplate}" />
  <Setter Property="togglDesktop:CheckBoxHelper.CheckBoxSize" Value="20" />
  <Setter Property="togglDesktop:CheckBoxHelper.CheckedBackgroundBrush" Value="{DynamicResource Toggl.AccentBrush}" />
  <Setter Property="togglDesktop:CheckBoxHelper.CheckedBorderBrush" Value="{DynamicResource Toggl.AccentBrush}" />
  <Setter TargetName="IntermediateBorder" Property="BorderBrush" Value="{Binding RelativeSource={RelativeSource TemplatedParent}, Path=(mah:ControlsHelper.FocusBorderBrush)}" />
  <Setter TargetName="UnCheckedBorder" Property="BorderBrush" Value="{Binding RelativeSource={RelativeSource TemplatedParent}, Path=(mah:ControlsHelper.FocusBorderBrush)}" />
  </Trigger>
  + <Trigger Property="Validation.HasError" Value="True">
  + <Setter TargetName="CheckedBorder" Property="BorderBrush" Value="{DynamicResource Toggl.ErrorMessageBackground}" />
  + <Setter TargetName="IntermediateBorder" Property="BorderBrush" Value="{DynamicResource Toggl.ErrorMessageBackground}" />
  + <Setter TargetName="UnCheckedBorder" Property="BorderBrush" Value="{DynamicResource Toggl.ErrorMessageBackground}" />
  + </Trigger>
  <Trigger Property="IsEnabled" Value="False">
  <Setter TargetName="CheckedBorder" Property="Background" Value="{DynamicResource Toggl.DisabledElementBrush}" />
  <Setter TargetName="CheckedBorder" Property="BorderBrush" Value="{DynamicResource Toggl.DisabledElementBrush}" />
  