bug#24579: [PATCH] borderless windows on OS X

[Jay McCarthy]

There are two popular Emacs patches for OS X that make all its windows
have no title bar:

https://github.com/nikhilunni/BorderlessEmacs

https://github.com/gwydirsam/emacs-mac-borderless

This patch integrates these by providing a variable
(ns-use-titled-windows) that allows this decision to be controlled at
runtime. The default is to use titles, like normal.

-- Some comments on the patch

This is my first Emacs patch. I hope it's okay.

The only difference between EmacsFSWindow and EmacsWindow is
explicitly telling OS X that the window can be the subject of focus,
which is required for borderless windows. (EmacsFSWindow has nothing
to do with being fullscreen.)

A toolbar can't be used without a title bar. If this isn't included,
then the console prints an error, but nothing bad happens. I think it
is a bit ugly to `if` out this, but not as ugly as an expected error
message.

I included a note in the Changelog, because it looks like all
variables like this get put in, but it could be removed from my
perspective... I have no need to satisfy my pride with my name in the
log. However, I didn't add anything to NEWS, because it seems to not
always mention things like this.

ns-use-titled-windows.patch
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