special alert on mention or /msg

[cburschka]

Many chat clients have a special sound if your name is mentioned in chat, or you receive a private message.

This is particularly useful if you're busy and want to turn other, less important sounds off.

[cburschka]

Moving up from 2.0-whenever to 1.4-sometimesoon.

[cburschka]

Patch is ready.

• Two new sound settings: "Private message" and "Alert".
• A 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.

(The order of 1., 2., 3. is based on the idea that a content-triggered alert is more significant than a message being private, but a message being private is more significant than it coming from a watched nickname.)