[Enhancement] Manage xfce4-notifyd notifications differently than notify-OSD ones

[Contribucious]

Generally speaking, when you send a notification (either in terminal by using notify-send or in other ways), it's managed differently according to the daemon used: notify-OSD for Unity/Gnome-Shell/Cinnamon/etc. and xfce4-notifyd for XFCE.

The default behavior for notify-OSD when we click on a notification is to open the program (Birdie) I think.
The default behavior for xfce4-notifyd when we click on a notification is to hide the notification and not open the program (Birdie).
(which is bad as, in addition, the links are not clickable with this daemon, and clicking anywhere on the notification don't bring the Birdie main window on foreground to let us click quickly on this link included in the notification)

Geary (mail client) seems therefore to have used a clever system that detects the notification daemon we use (maybe by detecting if the process xfce4-notifyd exists) to act differently for showing notifications.

I've Geary in stable version and I see that, only when using XFCE, the notification have a bonus button called "Open":
http://i.imgur.com/TF8izRx.png
=> So, click anywhere = default behavior (close the notification only)
=> And click on the "Open" button = open the program (to read the message quickly)

Can be good to act in the same way for Birdie notifications on XFCE too:
http://i.imgur.com/lbHzEwy.png
=> If the process xfce4-notifyd exists, just add a button "Open" below too, to show the Birdie main window when we click on it.
(here to bring the Birdie main window to click on this bit.ly link)