List: Default useTouchScroll to false on platforms with visible scrollbars

This disables the TouchScroll module's behavior on platforms where
has("touch") reports true, but discrete scrollbars are rendered.

This is an admittedly somewhat loose heuristic, but achieves in
preventing TouchScroll from hindering on platforms that typically
have mouse input available, such as IE and Chrome on hybrid Windows 8
laptop/tablet devices.  These platforms generally do just fine with
native touch scrolling support, as well.

Unfortunately, there is no known foolproof solution to feature-detect
the presence or absence of a mouse.