Add a hook run upon monitor configuration changes

Add a hook run upon monitor configuration changes

* doc/lispref/frames.texi (Multiple Terminals): Document new
hook `display-monitors-changed-functions'.
* etc/NEWS: Announce new abnormal hook.

* src/keyboard.c (kbd_buffer_get_event): Handle
MONITORS_CHANGED_EVENT.
(syms_of_keyboard): New hook and defsyms.
* src/termhooks.h (enum event_kind): Add new event
`MONITORS_CHANGED_EVENT'.

* src/xterm.c (handle_one_xevent): Handle RRNotify and
RRScreenChangeNotify events.
(x_term_init): Select for RRScreenChange, RRCrtcChange and
RROutputChange.
* src/xterm.h (struct x_display_info): Improve RandR version
detection.