notify_dbus replace previous notification with the same title

[drc0]

On linux, when using the dbus notification library, is possible to override the last notification in order to not have many notifications from the same user, I have a propose that looks at the current summary/title of the notifications, and following notifications with the same title are overridden, for example when receiving 3 notifications like: PM from me: "hello" PM from me: "world" PM from you: "!"

only two of them are displayed in the notification area, in particular those are shown:

PM from me: "world" PM from you: "!"

this avoids notifications floods, what do you think? ps (this pull request should be reviewed also with https://github.com/guyzmo/notossh/pull/9 because there the order of the title and body is inverted, line 116 )

thanks!