DatePicker
MetroCalendar - Default style
MetroCalendar - AccentColor and DefaultStyle
DatePicker - TextboxHelper.Watermark

TextboxHelper.Watermark  will override DatePickers internal Watermark,
which is less flexible...

Not working at designtime for some reason :-/
DatePicker - changed popup alignment

Previously aligned to internal textbox, which made the popup overlapp
the border.
Calendar - BlackBrush border to match textbox
added tab into demo app