Add icon buttons to notifications

Now if an id is passed to Notification.addButton that matches a theme icon,
it will be displayed as an icon.  This is another in the implement the
music player mockups series.
http://live.gnome.org/GnomeShell/Design/Guidelines/MessageTray/MusicPlayer

https://bugzilla.gnome.org/show_bug.cgi?id=621014

data/theme/gnome-shell.css

@@ -860,6 +860,19 @@ StTooltip {
     background: #808080;
 }
 
+.notification-icon-button {
+    border: 2px rgba(0,0,0,0.0);
+    border-radius: 5px;
+    padding: 5px;
+}
+.notification-icon-button:hover {
+    background: rgba(192,192,192,0.7);
+}
+
+.notification-icon-button:active {
+    background:  rgba(128,128,128,0.7);
+}
+
 .chat-received {
     background-gradient-direction: horizontal;
     background-gradient-start: #606060;