The notification spec supports the concept of a 'default' action:
"The default action (usually invoked my clicking the notification)
should have a key named "default". The name can be anything, though
implementations are free not to display it."
Support this by invoking the 'default' action rather than a emitting
the 'clicked' signal when clicking notifications which specifie a
default action.
Also don't add an action button for the default action.
https://bugzilla.gnome.org/show_bug.cgi?id=655818
ddd59f2e76