Introduce HstTagSupport base class

In more than one project, I have created a jsp tag extending from javax.servlet.jsp.tagext.TagSupport. I always had the need for methods similar to those in BaseHstComponent; therefore I copied methods like getDefaultClientComponentManager() and isPreview() to my own tag class.

Requesting to support those methods from HST itself, i.e. create a HstTagSupport class. Optionally a HstSimpleTagSupport too.

See HstTagSupport.java attached.