Ext.override：4种情况
如果target是使用Ext.define声明的一个类，给出overrides那个类的override方法被调用(看Ext.Base.override)
If the target is a class declared using Ext.define, the override method of that class is called (see Ext.Base.override) given the overrides.
如果target是一个函数，它假定是一个构造方法并且使用Ext.apply，overrides的内容被应用到它的原型上
If the target is a function, it is assumed to be a constructor and the contents of overrides are applied to its prototype using Ext.apply.
如果target是使用Ext.define声明的一个类的实例，overrides仅仅被应用到那个实例上。在这种情况下，方法被特殊处理去允许它们去使用Ext.Base.callParent。
If the target is an instance of a class declared using Ext.define, the overrides are applied to only that instance. In this case, methods are specially processed to allow them to use Ext.Base.callParent.
如果target不是这些，使用Ext.apply，overrides被应用到target。
If the target is none of these, the overrides are applied to the target using Ext.apply.