Expand spectrum of window change functions

Expand spectrum of window change functions

* src/window.c (run_window_change_functions): Run window
change functions for Qwindow_state_change_functions.
(resize_frame_windows): Set frame's window_change slot when
single-window frames change size.
(Qwindow_state_change_functions): New symbol.
(Vwindow_state_change_functions): New Lisp variable.
* doc/lispref/windows.texi (Selecting Windows): Mention
'window-selection/state-change-functions' and add reference to
Window Hooks.
(Window Hooks): Document 'window-state-change-functions'.
* etc/NEWS: Mention new hook 'window-state-change-functions'.