New items in direct message channels should ping if app is in background

[zarino]

If someone sends me a private message, and the app is in the background, I should be notified just as if they'd mentioned my name in a public channel.

That means an audible ping, and a red number on the dock icon.

Not sure whether there should also be a red number in the channelButton, or whether we should keep that for mentions of keywords only. I have a feeling, for consistency, there should be a red number in the channelButton, since otherwise, how am I meant to find out what triggered the beep that made me return to the app?

[zarino]

I guess we can check, inside ChannelView.addMessage(), whether the channel is a private message channel, by seeing whether fromUser == this.options.channelName.