Default theme: Improve compatibility with older browsers

Namely (it could hardly be different...) Internet Explorer - even IE11 still causes trouble. The default theme now supports IE9+, even older browsers will present broken markup. The sliding animation works with IE10+ (however, it is still usable, there's just no nice animation).

Furthermore this commit heavily improves the sliding process by allowing to abort the animation. I've updated Pico's screenshot in the , too.