(Qfixed_window_size): New.

(Qfixed_window_size): New.
(syms_of_window): Initialiaze it.
(check_all_windows): Add return type void.
(window_fixed_size_p): New.  Return non-zero if window
is fixed-size.
(window_min_size_1): New.
(window_min_size): Handle fixed-size windows.
(size_window): New. Rewritten combination of set_window_height and
set_window_width that handles fixed-size windows.
(set_window_height): Call it.
(set_window_width): Call it.
(Fsplit_window): Give an error on attempt to split a fixed-size
window.
(change_window_height): Partly rewritten to handle fixed-size
windows.