We really only care about messages from other people, so don't bother
notifying when messages are added to the WeeChat core buffer.

If you've received a highlight and a normal message comes in, or if you
have a private message and get highlighted, then the AnyBar colour
should not change. We should keep the AnyBar colour of the highest
priority message.

After using the plugin for a couple days, I decided that orange is too
insistent a colour to describe messages I probably don't need to read
immediately.

Swap the blue and orange colours so that now blue means regular message
and orange means private message. This feels better in terms of message
priority.

If a message comes in on a buffer whose notify level is set to
highlights and higher, then don't update AnyBar activity.

We should only be changing the AnyBar colour when we receive messages
that we want to be notified of based on our configuration.