Prevent open minibuffers getting lost when their frame gets deleted

Prevent open minibuffers getting lost when their frame gets deleted

This happened with minibuffer-follows-selected-frame set to t.

* doc/emacs/mini.texi (Basic Minibuffer): State where a command's action takes
place when a minibuffer's frame has been deleted.

* lisp/window.el (window--before-delete-windows, record-window-buffer): Take
into account that minibuffers are now recorded on w->prev_buffers field.

* src/fns.c (merge_c): New version of `merge' taking a C function, rather than
a Lisp function as the comparison function.

* src/frame.c (do_switch_frame): Pass arguments sf and for_deletion to
move_minibuffers_onnto_frame.

* src/lisp.h (top level): Declare merge_c and
move_minibuffers_onto_selected_frame.

* src/minibuf.c (MB_frame): New Lisp_Object recording the minibuffer's frame.
(choose_minibuf_frame): Remove all code except that which sets minibuf_window
to the current frame's minibuffer.
(minibuffer_ent_greater): New comparison function, passed to merge_c.
(zip_minibuffer_stacks): New function.
(move_minibuffers_onto_frame): Renamed from `move_minibuffer_onto_frame' given
two arguments, the old frame and for_deletion, and simplified.  Minibuffers
are now stacked in the mini-window's ->prev_buffers field.
(read_minibuf): Several detailed amendments.
(exp_MB_frame): New Lisp_Object, the expired minibuffer's frame.
(read_minibuf_unwind): Search for the expired minibuffer's frame, rather than
taking it from (unreliable) variables.  Switch temporarily to this frame for
tidying up operations.
(minibuffer_unwind): New function which pops a stacked minibuffer.
(syms_of_minibuf): Call staticpro for the two new Lisp variables.

* src/window.c (Fset_window_configuration): Don't record minibuffers with
record-window-buffer.

* src/xdisp.c (gui_consider_frame_title): Remove redundant Fselect_window,
which caused an unwanted frame switch.  Amend the arguments to
format_mode_line_unwind_data to match.