Validationerror example and improvements for issue #349

+ add simple validation example
* we don't need to set the Validation.ErrorTemplate to {x:Null}. only change the SnapsToDevicePixels to true to see a better border layout.
+ added example for ComboBox
! fix System.InvalidOperationException at ComboBox Style. remove the Storyboard.TargetName="validationTooltip" at <ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="IsHitTestVisible" Storyboard.TargetName="validationTooltip">
* LostFocus for UpdateSourceTrigger should also work