message: Add tooltip to time entry description (win)
diff: <TextBlock Grid.Row="0" Name="descriptionLabel" x:FieldModifier="private"
  MouseLeftButtonDown="onDescriptionLabelMouseDown"
  - Style="{DynamicResource Toggl.BodyText}" />
  + Style="{DynamicResource Toggl.BodyText}" >
  + <TextBlock.ToolTip>
  + <ToolTip DataContext="{Binding Path=PlacementTarget, RelativeSource={x:Static RelativeSource.Self}}">
  + <TextBlock Text="{Binding Text}"/>
  + </ToolTip>
  + </TextBlock.ToolTip>
  + </TextBlock>
  <TextBlock Grid.Row="0" Name="noDescriptionLabel" x:FieldModifier="private"
  Style="{DynamicResource Toggl.CaptionGrayishText}"
  Text="+ Add description"
  