Reorganize documentation relating to touch screens

Reorganize documentation relating to touch screens

Chiefly, elevate it from an appendix to a node in the User Input
chapter.  I have been approached time and again with questions
from people who have not searched the appendices for such
documentation.

* doc/emacs/emacs.texi: Move Other Input below Commands in the
menu.

* doc/emacs/input.texi (Other Input Devices): Rename to Other
Input.  All callers changed.
(Touchscreens, On-Screen Keyboards): Revise and reword
documentation.  Homogenize nomenclature for on screen keyboards,
preferring "virtual keyboards" after it has been mentioned once
by the other name.