HtmlBasedComponent supports addSclass() removeSclass()

When users adding customized style, they usually toggle a displaying effect by adding or removing a CSS class on a component.
Hence, API like addSclass() and removeSclass() are more convenient than setSclass and getSclass.

Spec are:

• public void addSclass(String cssCass)
  • append passed CSS class to a component's sclass property if the component doesn't have sclass.
  • If the component already has the CSS class sclass, do nothing.
• public void removeSclass(String cssClass)
  • remove passed CSS class from a component's sclass property if the component has sclass.
  • If the component doesn't have the CSS class sclass, do nothing.