Add scroll bar support to TAdvSmoothListBox

Please consider adding scrollbar support to TAdvSmoothListBox. Right now, the scroll position is shown but the user cannot actually scroll using the bar. You may want to look at Skype for Windows. They have a similar control and when you scroll it shows the position on the right like TAdvSmoothListBox does. But if you hover over that with the mouse it turns into a slightly wider bar and then you can use it to scroll up and down. It's a really nice, modern scrollbar implementation. Thanks!