new ImageCellRenderer to show pixbuf and pixbufAnimation in treeview (take less CPU time than the old method)

new preference option : icon in systray (changement is tacken into account at next start)

somedays read_queue would have to be a common share code and thus no specific
plugin code could be call from there.

(untested: not able to compile C module trayicon)

- icon in systray show that a new message has arrived if chat window in whitch it appears has not the focus (maybe it sould be an option ?)