Introduce an option which controls touch screen hscroll

Introduce an option which controls touch screen hscroll

* doc/emacs/input.texi (Touchscreens): Document this new
function and revise this node for clarity and pithiness.

* lisp/touch-screen.el (touch-screen): New custom group.
(touch-screen-display-keyboard, touch-screen-delay)
(touch-screen-precision-scroll, touch-screen-word-select)
(touch-screen-extend-selection, touch-screen-preview-select):
Move such options to that group.
(touch-screen-enable-hscroll): New user option.
(touch-screen-handle-scroll): If it is disabled, maintain the
hscroll accumulators but refrain from scrolling the window.