message: Split ValidationErrorTemplate into 2 parts for reusability
diff: xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
  xmlns:mah="http://metro.mahapps.com/winfx/xaml/controls">
  - <ControlTemplate x:Key="ValidationErrorTemplate">
  - <AdornedElementPlaceholder x:Name="placeholder">
  - <Grid SnapsToDevicePixels="True">
  - <Rectangle x:Name="PopupTargetElement"
  - Margin="-2 0"
  - DataContext="{Binding ElementName=placeholder}"
  - StrokeThickness="0" />
  - <Border x:Name="ValidationErrorElement"
  - BorderBrush="{DynamicResource Toggl.ValidationErrorBackground}"
  - BorderThickness="1"
  - DataContext="{Binding ElementName=placeholder}">
  - </Border>
  -
  - <mah:CustomValidationPopup x:Name="ValidationPopup"
  - Placement="Top"
  - AllowsTransparency="True"
  - IsOpen="False"
  - PlacementTarget="{Binding ElementName=PopupTargetElement, Mode=OneWay}">
  - <Grid x:Name="Root" Margin="5">
  + <Grid x:Key="ValidationPopupRoot" x:Shared="False" Margin="5">
  <Grid.Resources>
  <Style TargetType="Border">
  <Style.Triggers>
  - <DataTrigger Binding="{Binding ElementName=Root, Path=IsMouseOver, Mode=OneWay}" Value="True">
  + <DataTrigger Binding="{Binding Path=IsMouseOver, RelativeSource={RelativeSource AncestorType={x:Type Grid}}, Mode=OneWay}" Value="True">
  <DataTrigger.EnterActions>
  <BeginStoryboard>
  <Storyboard>
  <ItemsControl ItemsSource="{Binding}" />
  </Border>
  </Grid>
  +
  + <ControlTemplate x:Key="ValidationErrorTemplate">
  + <AdornedElementPlaceholder x:Name="placeholder">
  + <Grid SnapsToDevicePixels="True">
  + <Rectangle x:Name="PopupTargetElement"
  + Margin="-2 0"
  + DataContext="{Binding ElementName=placeholder}"
  + StrokeThickness="0" />
  + <Border x:Name="ValidationErrorElement"
  + BorderBrush="{DynamicResource Toggl.ValidationErrorBackground}"
  + BorderThickness="1"
  + 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>
  