More color controls for nav menu

[cristianraiber]

• [x] highlight active menu item & drop-down color should be separate color pickers/values
• [x] item color in drop-down needs to have a separate color picker (right now, hover item color can only have one color that's used for both drop-down menus and "main" nav menu [ not a dropdown ] ).
• [x] border-bottom color
• [x] background color for nav menu when sticky-ed