Rename window-nest to window-combination-limit.

Rename window-nest to window-combination-limit.

* window.h (window): Rename slot "nest" to "combination_limit".
* window.c (Fwindow_nest): Rename to Fwindow_combination_limit.
(Fset_window_nest): Rename to Fset_window_combination_limit.
(Vwindow_nest): Rename to Vwindow_combination_limit.
(recombine_windows, make_parent_window, make_window)
(Fsplit_window_internal, saved_window)
(Fset_window_configuration, save_window_save): Rename all
occurrences of window_nest to window_combination_limit.
* window.el (split-window, window-state-get-1)
(window-state-put-1, window-state-put-2): Rename occurrences of
window-nest to window-combination-limit.
* cus-start.el (window-nest): Rename to
window-combination-limit.
* windows.texi (Resizing Windows, Splitting Windows): Rename
occurrences of window-nest to window-combination-limit.