notifications: increase timeout of connection approve notification
As for now Vino doesn't set any timeout of connection prompt notification.
This is not real problem (rather, this is an inconvenience) for GNOME Shell since it supports persistent notifications, however for other DEs without persistent notifications support this is lead to inability to accept remote
connection at all.
This patch sets 3 minute notification timeout which will allow to react in time to the notification (for DEs that supports notification timeout).