Unify Fx_focus_frame between all ports.

Unify Fx_focus_frame between all ports.
* src/frame.h (x_focus_frame): New prototype.
* src/xfns.c (Fx_focus_frame): Remove.
(syms_of_xfns): Do not defsubr it.
(x_focus_frame): X implementation.
* src/nsfns.m (Fx_focus_frame): Remove.
(syms_of_nsfns): Do not defsubr it.
(x_focus_frame): NS implementation.
* src/w32term.c (Fx_focus_frame): Remove.
(x_focus_on_frame): Rename to...
(x_focus_frame): W32 implementation.
* src/w32term.h (x_focus_on_frame): Remove prototype.
* src/w32fns.c (Fx_focus_frame): Remove.
(syms_of_w32fns): Do not defsubr it.
* src/frame.c (Fx_focus_frame): Define here.
(syms_of_frame): Defsubr here.
* src/gtkutil.c (xg_tool_bar_callback): Use x_focus_frame.
* lisp/frame.el (x-focus-frame): Mark as declared in frame.c.