Refine notification style

Update the notification style according to the latest mockups:
 - make notification icons smaller and prefer symbolic variants
 - remove background box when not hovered/focused
 - increase spacing between elements
 - use normal text sizes

https://bugzilla.gnome.org/show_bug.cgi?id=775763

js/ui/messageList.js

@@ -318,6 +318,7 @@ const Message = new Lang.Class({
 
         this._iconBin = new St.Bin({ style_class: 'message-icon-bin',
                                      y_expand: true,
+                                     y_align: St.Align.START,
                                      visible: false });
         hbox.add_actor(this._iconBin);