苹果文档说明：
To participate in the appearance proxy API,tag your appearance
property selectors in your header with UI_APPEARANCE_SELECTOR.
在Objective-C中，可以使用UI_APPEARANCE_SELECTOR注释属性，如下所示：
@property (nonatomic,strong) UIColor *foregroundColor UI_APPEARANCE_SELECTOR;
我怎么能在Swift中做同样的事情？
