toolkit: Refactor document structures into builders and description files

Goal is to further increase the shared code between the standard doclet and
MIF doclet, reducing their discrepancies, making the doclets smaller and
simpler. This would also make it easier to generate new doclets.
We could refactor the code further in two ways:

- Abstract out document structures such as headings, paragraphs, bulleted lists,
  indented paragraphs, tables, code paragraphs, and so forth into builders in
  the toolkit shared code. The actual markup for these would be provided by the
  individual doclets, including the formatting such as fonts, colors,
  alignment, positioning, margins, amount of indent, and so forth.

- Abstract out the order of elements on a page into page description files
  that are markup-independent. These then become shared code that
  is part of the toolkit. A set of page description files could be used
  by any doclet. This would ensure that the order is identical for
  different doclets, and would enable easy modification of the output
  when Javadoc is run. These files would likely be in XML.

When generating a class page, for instance, the HTML doclet would specify
a particular set of description files, then call the toolkit for each page
element. It might call the toolkit for a table version of the member
summary, and the toolkit would in turn ask the HTML doclet for the markup
to start a table, start a row, start a cell, etc.

Taken to the limit, the individual doclets (HTML, MIF, RTF) are simply drivers
that specify a set of description files and contain markup definitions
(e.g., for HTML, paragraph start is <P>). James Gosling is a big proponent
of this approach.

The main descriptions files would be:

Main pages:
   Overview Description File
   Package Description File
   Class Description File
   PackageTree Description File
   ClassTree Description File
   PackageUse Description File
   ClassUse DescriptionFile
   Deprecated DescriptionFile
   SerializedForm Description File
   ConstantValues Description File
   IndexSingle Description File
   IndexSplit Description File

Frames:
   Frameset Description File
   Overview-Frame Description File
   Package-Frame Description File
   AllClasses-Frame Description File

For the MIF Doclet, there would be additional files:
   Book File (effectively, today's XML Book File)
   TitlePage Description File
   AlmanacLegend Description File
   Almanac Description File
   CrossReference Description File