macOS: implement bell support

[jcollie]

#7087 implemented support for the BELL character on GTK (and added infrastructure to the core). #5326 will add support for custom audio for GTK. Bell support was originally discussed in #2710 .

[ftpd]

Will this also cover the icon badge?

[mitchellh]

There are two really basic APIs we can look at to follow GTK's path:

• NSApplication.requestUserAttention(_:)
• NSBeep

I think we should just do the first. I think the beep is universally hated on macOS so there's no reason to even support it. If and when we support audio bells with custom audio and so on we can have an option for the system beep.

macOS doesn't have the concept of a tab or window attention, it's only at the app scope. So if we want to do more focused attention, then we should probably implement a bell in the title or something.

[mitchellh]

Will this also cover the icon badge?

Is that this?

https://developer.apple.com/documentation/appkit/nsdocktile/badgelabel

That could also be a consideration, but I wasn't able to get this API to work personally.

[ftpd]

Is that this?

I can't say, I'm just an end-user without any knowledge of writing apps for macOS. What I would like to have is something like that (screen is from macOS' Terminal.app). Easiest way to trigger it is doing tput bel in the shell.