Mousewheel-driven smooth scrolling support.

This is based on nice experimental logic once made by Allan, that was later dubiously integrated by various vendors. The timings were revisited to enhance responsiveness, aiming for a feeling close to FireFox's.

We support two main enabling modes : unconditional (Enabled), and energy-conscious (WhenEfficient) - that will avoid smooth scrolling pages with static elements.

Improvements to scrolling efficiency for static elements will make the worst-case in Enabled mode quite rare, but still not unlikely.

WhenEfficient is thus the recommended mode.