Pan during touch screen pinch gestures

Pan during touch screen pinch gestures

* doc/lispref/commands.texi (Touchscreen Events): Document new
ratio-diff parameter to touchscreen-pinch events.

* lisp/completion-preview.el (completion-preview-commands):
Account text-conversion events preview commands as well.

* lisp/touch-screen.el (touch-screen-aux-tool): Introduce two
new elements.
(touch-screen-pinch): Scroll window in accord with event deltas.
(touch-screen-handle-aux-point-update): Supply the ratio
difference in generated events.
(touch-screen-handle-touch): Create a vector with those two new
elements.