Make balloons appear only once when receiving a lot of messages

[amroth]

KMess should avoid showing a balloon every time a chat message is receiving; with multiple conversations open, it becomes very annoying having a balloon to appear for every text message received from each contact.

A balloon should be shown only when you receive the first chat message from a non-focused window.

Messages of other types, like invitations, should always appear; so should do online/offline balloons. Eemail notifications should appear once too, only one for sender address, for example... But there would be impossible to know when the previous messages have been read and KMess can show again a balloon.