Move some window-related functions from frame.c to window.c.

Move some window-related functions from frame.c to window.c.

* lisp.h: Move EXFUNS for Fframe_root_window,
Fframe_first_window and Fset_frame_selected_window to window.h.

* window.h: Move EXFUNS for Fframe_root_window,
Fframe_first_window and Fset_frame_selected_window here from
lisp.h.

* frame.c (Fwindow_frame, Fframe_first_window)
(Fframe_root_window, Fframe_selected_window)
(Fset_frame_selected_window): Move to window.c.
(Factive_minibuffer_window): Move to minibuf.c.
(Fother_visible_frames_p): New function.

* minibuf.c (Factive_minibuffer_window): Move here from frame.c.

* window.c (Fwindow_frame): Move here from frame.c.  Accept any
window as argument.
(Fframe_root_window, Fframe_first_window)
(Fframe_selected_window): Move here from frame.c.  Accept frame
or arbitrary window as argument.  Update doc-strings.
(Fminibuffer_window): Move up in code.
(Fwindow_minibuffer_p): Move up in code and simplify.
(Fset_frame_selected_window): Move here from frame.c.  Marginal
rewrite.
(Fselected_window, select_window, Fselect_window): Move up in
code.  Minor doc-string fixes.