Improve touch screen support

Improve touch screen support

* doc/emacs/input.texi (Touchscreens): Document the new feature
for people who have trouble dragging to word boundaries.
* lisp/touch-screen.el (touch-screen-word-select): New
defcustom.
(touch-screen-word-select-bounds)
(touch-screen-word-select-initial-word): New variable
definitions.
(touch-screen-hold): If `touch-screen-word-select', select the
word around EVENT.
(touch-screen-drag): If `touch-screen-word-select', extend the
region to the next word boundary if the character under point
constitutes a word.
(touch-screen-handle-point-update, touch-screen-handle-touch)
(touch-screen-translate-touch): Fix doc strings and fill
comments.