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
updated TitleForeground to use converter
returned TitleForeground as per AmryKid
Merge pull request #634 from Icehunter/groupbox-header-fix

Update Controls.GroupBox.xaml