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.
punker76 fix for groupbox-header
punker76 update for header

• added BackgroundToForegroundConverter
Fixed MetroDemo.NET45 failure to compile due to missing CleanWindowDemo.xaml (and .xaml.css).
updated TitleForeground to use converter
Changed StaticResource to DynamicResource for RadioButton

My application kept crashing because RadioButton was using a Static instead of a DynamicResource, I think where it can be Dynamic it should be Dynamic.
Merge pull request #641 from stefan-schweiger/patch-3

Changed StaticResource to DynamicResource for RadioButton
Merge pull request #643 from Amrykid/fix-everything

Added missing CleanWindowDemo.xaml.
returned TitleForeground as per AmryKid
Merge pull request #634 from Icehunter/groupbox-header-fix

Update Controls.GroupBox.xaml