MessageTray: rework icon handling to split model and view

To allow more than one summary icon actor for a source we split
the model of the source icon (which is iconName, if the default
implementation is used, or a GIcon otherwise) and replace
createNotificationIcon() with a generic createIcon(size). Also,
the actual source actor is split into a separate class, that handles
the notification counter automatically.

https://bugzilla.gnome.org/show_bug.cgi?id=619955

js/ui/windowAttentionHandler.js

@@ -73,8 +73,8 @@ const Source = new Lang.Class({
         this.signalIDs = [];
     },
 
-    createNotificationIcon : function() {
-        return this._app.create_icon_texture(this.ICON_SIZE);
+    createIcon : function(size) {
+        return this._app.create_icon_texture(size);
     },
 
     open : function(notification) {