Share systray tooltip between EIP and mail

EIP status does not update properly in the systray tooltip, it gets stuck in "retrieving configuration"... Mail status is not working, there is a comment in @mail_status.py:#173:_update_systray_tooltip@ explaining that is disabled.

After splitting the status panel into eip_status and mail_status, the tray tooltip stopped working correctly. We should implement a shared systray manager or something.

(from redmine: created on 2013-10-02, closed on 2013-11-11, relates #3758 (closed), blocks #3927 (closed))