Improve touch-screen support

Improve touch-screen support

* doc/emacs/emacs.texi (Top):
* doc/emacs/input.texi (Other Input Devices): Correctly
capitalize subsection name.
(Touchscreens): Document additional translation.
* doc/lispref/commands.texi (Touchscreen Events): Document that
`touchscreen-end' events now have prefix keys.  Also, describe
mouse emulation and `touchscreen-scroll' events.
* doc/lispref/keymaps.texi (Translation Keymaps): Document
`current-key-remap-sequence'.
* lisp/touch-screen.el (touch-screen-translate-prompt): New
function.
(touch-screen-scroll): New command.  Bind to
`touchscreen-scroll'.
(touch-screen-handle-point-update, touch-screen-handle-point-up)
(touch-screen-handle-touch): Refactor to actually translate
touch screen event sequences, as opposed to looking up commands
and executing them.
(touch-screen-translate-touch): New function.  Bind in
function-key-map to all touch screen events.
(touch-screen-drag-mode-line-1, touch-screen-drag-mode-line)
(touch-screen-tap-header-line): Remove special commands for
dragging the mode line and clicking on the header line.
* lisp/wid-edit.el (widget-button-click): Adjust accordingly.
* src/keyboard.c (access_keymap_keyremap): Bind
`current-key-remap-sequence' to the key sequence being remapped.
(keyremap_step): Give fkey->start and fkey->end to
access_keymap_keyremap.
(head_table): Add imaginary prefix to touchscreen-end events as
well.
(syms_of_keyboard): New variable Vcurrent_key_remap_sequence.