Add an option to display the classes in inheritance order, rather than
alphabetic order, in the left-hand frame. This would be useful for
small APIs, such as the Doclet API. You would get this:

Doclet
  Doc
    PackageDoc
    ProgramElementDoc
       ClassDoc (also extends Type)
       MemberDoc
         ExecutableMemberDoc
           ConstructorDoc
           MethodDoc
         FieldDoc
    RootDoc (also extends DocErrorReporter)
  SerialFieldTag (also extends Tag)

Type
  ClassDoc (also extends ProgramElementDoc)

DocErrorReporter
  RootDoc (also extends Doc)

Parameter

SourcePosition

Tag
  ParamTag
  SeeTag
  SerialFieldTag
  ThrowsTag

Notice some terms are duplicated, due to multiple
implmenting of interfaces. We might want to remove
the duplication, which would produce:

-------------------------------------------
Doclet
  Doc
    RootDoc (also extends DocErrorReporter)
    PackageDoc
    ProgramElementDoc
       ClassDoc (also extends Type)
       MemberDoc
         ExecutableMemberDoc
           ConstructorDoc
           MethodDoc
         FieldDoc
Type

DocErrorReporter

Parameter

SourcePosition

Tag
  ParamTag
  SeeTag
  SerialFieldTag (also extends Doc)
  ThrowsTag
-------------------------------------------
I arrived at this list by deciding to keep SerialFieldTag under
Tag, and keep ClassDoc under Doc. This can only be done by user
intervention. Perhaps the order of multiple inheritance interfaces
needs to be specified by the user?

In any case, this is more meaningful than the current alphabetic list:

 ClassDoc
 ConstructorDoc
 Doc
 DocErrorReporter
 Doclet
 ExecutableMemberDoc
 FieldDoc
 MemberDoc
 MethodDoc
 PackageDoc
 ParamTag
 Parameter
 ProgramElementDoc
 RootDoc
 SeeTag
 SerialFieldTag
 SourcePosition
 Tag
 ThrowsTag
 Type