Update Controls.GroupBox.xaml

Using dark themes makes text in header as black which is difficult to see. Hard coded much like TitleForeground for MetroWindow.

LightTheme:
![2013_08_28_09 17 27_](https://f.cloud.github.com/assets/677341/1043414/9295c2be-0ffd-11e3-9911-683f115540fe.jpg)
Old DarkTheme:
![2013_08_28_09 18 45_](https://f.cloud.github.com/assets/677341/1043416/92bf151a-0ffd-11e3-9638-f410c8f22e9b.jpg)
New DarkTheme:
![2013_08_28_09 21 58_](https://f.cloud.github.com/assets/677341/1043445/067e219e-0ffe-11e3-95e2-0239c419fe78.jpg)