Flyouts improvement

+ add MetroLabel style
+ improve/update style for left and right flyouts
+ fix foreground bug (force showing WhiteBrush color)
+ add more examples for right and left flyouts
+ add FlyoutWhiteBrush for dark and light accents
+ change foreground of checkbox, label, radiobutton and textblock to FlyoutWhiteColor / FlyoutWhiteBrush