Expose the name of an event's input device to Lisp

Expose the name of an event's input device to Lisp

This name can be used to identify the device for special
treatment, i.e. only interpolating scrolls coming from mice and
not touchpads inside pixel-scroll-precision-mode.

* doc/lispref/commands.texi (Command Loop Info): Document new
variable `last-event-device'.
* etc/NEWS: Announce new variable `last-event-device'.
* src/frame.h (struct frame): New field `last_mouse_device'.
* src/keyboard.c (read_char): Clear last-event-device.
(kbd_buffer_get_event): Set last-event-device to the event's
recorded device.
(init_keyboard): Clear last-event-device.
(syms_of_keyboard): New defvar `last-event-device'.
* src/termhooks.h (struct input_event): New field `device'.
(EVENT_INIT): Set it to the special value `Qt' by default.
* src/xterm.c (x_init_master_valuators): Record the device's
name.
(x_dnd_begin_drag_and_drop): Only preserve last event device if
the mouse ended up in the source frame.
(x_note_mouse_movement): New argument `source'.
(handle_one_xevent): Set input event sources whenever
appropriate.
(mark_xterm): Mark device names.

* src/xterm.h (struct xi_device_t): New field `name'.