Allow minibuffer to stay in its original frame. Tidy up this area.

Allow minibuffer to stay in its original frame.  Tidy up this area.

* doc/emacs/mini.texi (Basic Minibuffer): Add an entry for
minibuffer-follows-selected-frame.

* doc/lispref/minibuf.texi (Minibuffer Misc): Describe the new parameter to
minibufferp, LIVE.

* etc/NEWS: Add an entry describing the new minibuffer strategy.

* lisp/cus-start.el (minibuffer-prompt-properties--setter): Add an entry for
minibuffer-follows-selected-frame.

* lisp/minibuffer.el (minibuffer-message): Check for the current buffer being
an _active_ minibuffer rather than merely a minibuffer.

* src/frame.c (do_switch_frame): Call move_minibuffer_onto_frame.

* src/lisp.h (Top level): Add prototypes for move_minibuffer_onto_frame and
is_minibuffer.

* src/minibuf.c (minibuf_follows_frame): New function which ignores local and
let-bound values of minibuffer-follows-selected-frame.
(choose_minibuf_frame): Reformulate this function to reuse a minibuffer window
where possible, and to ensure no other frame has its minibuffer current, but
only when `minibuffer-follows-selected-frame'.
(move_minibuffer_onto_frame): New function.
(live_minibuffer_p): New function.
(Fminibufferp): Add a new &optional parameter LIVE.  Reformulate, possibly
calling live_minibuffer_p.
(read_minibuf): move the incrementation of minibuf_level to before the call of
choose_minibuf_frame.  Empty the miniwindows of frames without an active
minibuffer, rather than of all but the current frame.
(is_minibuffer): New function.
(read_minibuf_unwind): Note the miniwindow being restored and resize all other
miniwindows to zero size.
(minibuffer-follows-selected-frame): New configuration variable.

* src/window.c (candidate_window_p): In some scenarios, check the miniwindow
holds an active minibuffer.

* src/xdisp.c (get_window_cursor_type): Suppress the cursor for non-active
miniwindows, regardless of minibuf_level.