Create CHANNEL_SELECT IPC event

[RecklessBoon]

It would be great to have an event fired when a text channel is selected or left. This would allow my external app to track if a user has possibly seen notifications from the NOTIFICATION_CREATE event. Currently there is no way to follow up when a cached notification should be removed. Adding this event will at least provide a semi-reliable way of knowing when the user has come across their new message/mention in a text channel.