Improve touch-screen support

Improve touch-screen support

* doc/lispref/commands.texi (Touchscreen Events): Document
changes.
* lisp/touch-screen.el (touch-screen-current-tool): Update doc
string.
(touch-screen-precision-scroll): New user option.
(touch-screen-handle-scroll): Use traditional scrolling by
default.
(touch-screen-handle-touch): Adust format of
touch-screen-current-tool.
(touch-screen-track-tap): Don't print waiting for events.
(touch-screen-track-drag): Likewise.  Also, don't call UPDATE
until threshold is reached.
(touch-screen-drag-mode-line-1, touch-screen-drag-mode-line):
Improve window dragging.