bug#20481: 24.5; Newlines in message-box output don't work on Windows

[Eli Zaretskii]

> From: Adam Connor <adamc55@gmail.com>
> Date: Thu, 30 Apr 2015 22:18:13 -0500
> 
> If I display a message using message-box, newlines seem to be ignored.
> For example
> (message-box "secret\nmessage")
> 
> will show the message "secretmessage". I have also tried \r\n with the
> same result.
> 
> Not a huge problem but annoying...

Emacs on Windows doesn't really support message boxes (a.k.a. "dialog
boxes"), except for simple Yes/No questions popped up by yes-or-no-p.
For other messages, such as this one, Emacs on Windows emulates dialog
boxes by menus, and in a menu an item cannot contain a newline.

In this case, we faithfully pass the original string with a newline to
the MS-Windows menu API, and I believe it is dropped by Windows when
it displays the menu.

IOW, this is a missing feature: dialog boxes are not fully supported
on MS-Windows.