Fix GroupBox ControlTheme + Respect BorderBrush, BortherThickness, and Foreground

[ScrubN]

• Achieves a similar goal as Fixes GroupBox lack of ControlTheme applied #563
• Adds a Border to the template so the GroupBox respect it's BorderBrush, BorderThickness, and Background properties
• I wanted to make the Foreground brush apply to the header as well, but because there is no way to conditionally apply a style if a property is set to the default value, I could not make it happen :(