macOS: Allow hiding the titlebar only instead of all window decorations

[Luflosi]

Don't merge this yet, I haven't written the documentation yet, I just wanted to get some feedback first.

This commit un-deprecates the macos_hide_titlebar config option and implements it how it was supposed to function in the first place.

Thanks to @ntruong for writing ntruong@cb8279e and ntruong@6807411, the code was very helpful.

Is it ok to un-deprecate this config option and change what it does?
Is it ok to call cocoa_hide_window_title() in cocoa_hide_titlebar() or does that cause any problems with the autoreleasepool blocks?

hide_window_decorations yes:

macos_hide_titlebar yes:

[kovidgoyal]

Rather than undeprecate the option make hide_window_decorations take a titlebar-only keyword. It should be fine as far as I know autorelease pools can be arbitrarily nested.

[Luflosi]

But this only works on macOS, which is why I think it is not a good idea to use hide_window_decorations for this.

[kovidgoyal]

That option already has a disclaimer saying actual effects are OS/window
manager dependant. Simply add to that disclaimer saying titlebar-only
works only on macOS.

[Luflosi]

Like this? Keep in mind that I still did not change the documentation yet.

[kovidgoyal]

Looks fine.

[Luflosi]

Please review.