Trigger change event for add/remove

Trigger change event for add_widget / remove_widget methods and refactor
the trigger into a method for uniformity

README.md

@@ -158,7 +158,7 @@ to completely lock the widget.
 
 ### onchange(items)
 
-Occurs when widgets change their position/size
+Occurs when adding/removing widgets or existing widgets change their position/size
 
 ```javascript
 var serialize_widget_map = function (items) {

src/gridstack.js

@@ -509,6 +509,21 @@
         $(window).resize(on_resize_handler);
         on_resize_handler();
     };
+    
+    GridStack.prototype._trigger_change_event = function(forceTrigger) {
+        var elements = this.grid.get_dirty_nodes();
+        var hasChanges = false;
+
+        var eventParams = [];
+        if (elements && elements.length) {
+            eventParams.push(elements);
+            hasChanges = true;
+        }
+
+        if (hasChanges || forceTrigger === true) {
+            this.container.trigger('change', eventParams);
+        }
+    };
 
     GridStack.prototype._init_styles = function() {
         if (this._styles_id) {
@@ -638,9 +653,7 @@
                 .attr('data-gs-height', node.height)
                 .removeAttr('style');
             self._update_container_height();
-            var elements = self.grid.get_dirty_nodes();
-            if (elements && elements.length)
-                self.container.trigger('change', [elements]);
+            self._trigger_change_event();
 
             self.grid.end_update();
         };
@@ -704,6 +717,7 @@
         this.container.append(el);
         this._prepare_element(el);
         this._update_container_height();
+        this._trigger_change_event(true);
 
         return el;
     };
@@ -722,6 +736,7 @@
         this._update_container_height();
         if (detach_node)
             el.remove();
+        this._trigger_change_event(true);
     };
 
     GridStack.prototype.remove_all = function(detach_node) {
@@ -846,9 +861,7 @@
         callback.call(this, el, node);
 
         self._update_container_height();
-        var elements = self.grid.get_dirty_nodes();
-        if (elements && elements.length)
-            self.container.trigger('change', [elements]);
+        self._trigger_change_event();
 
         self.grid.end_update();
     };