XfceAboutDialog missing getter and setter

[Jeff Franks]

Benny,

There is no getter and setter for the XfceAboutDialog icon property. 
Since xfce_about_dialog_new_empty() creates an empty about dialog there 
should be. Also I notice that xfce_about_dialog_new_with_values() does 
not call g_object_ref() to reference the icon but 
xfce_about_dialog_finalize() calls g_object_unref() to unreference the icon.

Jeff.