Remove term "subwindow" from code and documentation.

Remove term "subwindow" from code and documentation.

* window.h: Declare delete_all_child_windows instead of
delete_all_subwindows.
* window.c (Fwindow_nest, Fset_window_nest)
(Fset_window_new_total, Fset_window_new_normal)
(Fwindow_resize_apply): Don't use term subwindow in doc-strings.
(delete_all_subwindows): Rename to delete_all_child_windows.
(Fdelete_other_windows_internal, Fset_window_configuration):
Call delete_all_child_windows instead of delete_all_subwindows.
* frame.c (delete_frame): Call delete_all_child_windows instead
of delete_all_subwindows.

* window.el (window-combinations): Make WINDOW argument
mandatory.  Rewrite doc-string.
(walk-window-subtree, window-atom-check, window-min-delta)
(window-max-delta, window--resize-this-window)
(window--resize-root-window-vertically, window-tree)
(balance-windows, window-state-put): Rewrite doc-strings as to
not mention the term "subwindow".
(window--resize-subwindows-skip-p): Rename to
window--resize-child-windows-skip-p.
(window--resize-subwindows-normal): Rename to
window--resize-child-windows-normal.
(window--resize-subwindows): Rename to
window--resize-child-windows.
(window-or-subwindow-p): Rename to window--in-subtree-p.

* windows.texi (Resizing Windows, Splitting Windows)
(Window Configurations): Use "child window" instead of
"subwindow".