Handle removal of selected tty child frame

Handle removal of selected tty child frame

* src/dispextern.h (root_frame):
* src/frame.h (root_frame): Move declaration from dispextern.h
to frame.h.
(SET_FRAME_VISIBLE): Whend making the selected tty child frame
invisible, use mru_rooted_frame to find a frame to switch to.
* src/dispnew.c (root_frame): Move root_frame to frame.c.
* src/frame.c (do_switch_frame): On ttys don't change the
top frame when switching from a child frame to another frame
with the same root.
(root_frame): Move here from dispnew.c.
(Fframe_root_frame): New Lisp function.
(delete_frame): Whend deleting the selected tty child frame use,
mru_rooted_frame to find a frame to switch to.
* src/window.c (mru_rooted_frame): New function.
* src/window.h (mru_rooted_frame): Declare it.
* doc/lispref/frames.texi (Child Frames): Describe new function
'frame-root-frame'.

(cherry picked from commit d4aeb6bd230c42cf7b773ec9aebd80ad6d928d98)