network-agent: Allow entries to activate default action

Currently entries' 'activate' signal is ignored, so hitting enter
does not have any effect, even if all required information has been
entered.
Instead, connect to the 'activate' signal so that hitting enter
behaves as if the "OK" button had been pressed.

https://bugzilla.gnome.org/show_bug.cgi?id=659133

js/ui/networkAgent.js

@@ -115,6 +115,7 @@ NetworkSecretDialog.prototype = {
                     initialFocusSet = true;
                 }
 
+                secret.entry.clutter_text.connect('activate', Lang.bind(this, this._onOk));
                 secret.entry.clutter_text.connect('text-changed', Lang.bind(this, function() {
                     secret.value = secret.entry.get_text();
                     if (secret.validate)