ctor: after creation, add this as a listener to a BookDataDisplay, for
example: basic/TextPaneBookDataDisplay.java
This class can also have a list of URIEvent content retrievers, who
specialize in retrieving the content of a URI request and may also
perform some kind of processing, such as converting to html (return a
string if success, null o/w):
Such specialized retrievers will have two methods: public boolean
handles(String protocol); public String retrieve(URIEvent evt);