New variable set-message-function to show message at the end of the minibuffer

New variable set-message-function to show message at the end of the minibuffer

* doc/lispref/display.texi (Displaying Messages): Document
set-message-function and clear-message-function.

* lisp/minibuffer.el (minibuffer-message-clear-timeout): New defcustom.
(minibuffer-message-timer, minibuffer-message-overlay): New variables.
(set-minibuffer-message, clear-minibuffer-message): New functions.
(set-message-function, clear-message-function): Set variables to
set-minibuffer-message and clear-minibuffer-message respectively.

* src/keyboard.c (read_char): Call clear_message when
Vclear_message_function is a function.

* src/xdisp.c (set_message): Call Vset_message_function when it's a function.
(clear_message): Call Vclear_message_function when it's a function.
(syms_of_xdisp): New variables set-message-function and clear-message-function
(bug#38457).