Fix #65: Alert on mention, keyword and msg.

* Two new sound settings: "Private message" and "Alert".
* New setting: "Alert keywords". These keywords apply to the message
  content, but also to the sender nickname to allow monitoring
  a particular user.

* If a message arrives, it triggers the first applicable and enabled
  sound effect in this order:

  1. Message contains user's nickname or an alert keyword: "Alert".
  2. Private message: "Private message".
  3. Sender nickname contains an alert keyword: "Alert".
  4. Otherwise: "Incoming message", as before.