3.6: added FastScrollOverScrollbar advanced setting

[kjk]

In latest 3.6 pre-release I've added FastScrollOverScrollbar advanced setting.

If set to true, when mouse is over a scrollbar, scrolling with mouse wheel is faster.

For backwards compatibility it's set to false.

This has been enabled by default in pre-release builds for a while but people reported that it can be triggered by accident, which is confusing, so I decided to require explicit opt-in.