Update MahApps.Metro/Themes/Panorama.xaml

The HeaderFontColor binding was not always working. And the problem and solution are described at http://stackoverflow.com/questions/4665645/cannot-override-labels-style-in-wpf
Update MahApps.Metro/Controls/Panorama.cs

Clicking a selected item should deselect it (i.e. toggle). This is particularly useful when the Panorama control is used to select multiple items.
Fixed GroupBox foreground color issue.
Merge branch 'master' of github.com:MahApps/MahApps.Metro