updated TextboxHelper.Watermark to respect Textbox Padding

When I tried adding padding to a TextBox while using TextboxHelper.Watermark, the watermark does not respect the padding value, making it awkward and out of place.

![](http://i.imgur.com/CtXLfjN.png)

I've updated the styles for the watermark TextBlock to respect the TextBox's padding value. Hope that works!