A callback function that will be invoked each time the active menu item in the button's menu has changed. If this
callback is not supplied, the SplitButton will instead fire the change event on active item change. The
changeHandler function will be called with the following argument list: (SplitButton this, Ext.menu.CheckItem
A css class which sets an image to be used as the static icon for this button. This icon will always be displayed
regardless of which item is selected in the dropdown list. This overrides the default behavior of changing the
button's icon to match the selected item's icon on change.