Added the Floating Watermark feature

Adds the Floating Watermark feature.

Add ```Controls:TextboxHelper.UseFloatingWatermark="True"``` to a TextBox/PasswordBox with a watermark and it will appear when there is text in the box. The foreground color will be the box's, unless the box is focused, then it will have the Foreground brush set to AccentColorBrush.

Closes #1318.