Improve scrollbars in tabstrips

[rock3r]

Scrollbar behaviour and appearance in tab strips seems completely custom. It may be worth having a separate ScrollbarVisibility just for them, or even a completely custom component.

1. The scrollbar appears immediately when the mouse hovers the tabstrip
2. The scrollbars are thin, but get thicker and darker (without animation) when directly hovered
3. Appearance and disappearance are animated (fade in/out)
4. I think (need to verify) that these scrollbars look the same on all OSes