major cleanup of relax ng schema for HTML
notes, schema improvements

 * schema changes
   * moved rearnotes to bodymatter
   * fixed allowed class values for cover and flaps
   * allow nested lists
   * allow pagebreak in list item block context
   * added render-required and render-optional to list of classes (for easier content completion in editors)
   * replaced all references to "z3998:covertitle" with "covertitle"
   * HTML: removed much of tpb123 which is mostly relevant for DTBook
   * HTML: disabled tpb134
   * disabled validation of rearnote and footnote placement while waiting for #160
   * require class="title" on fulltitle
   * require class="docauthor" on main author
 * properly mapped footnotes and rearnotes which fixes #161 (at least in terms of DTBook/HTML; HTML splitting still remains, see #160)
 * note is mapped to either aside or list item depending on what kind of note it is (rearnote/footnote becomes list item)
 * adding notebody class to all notes in HTML
 * put list marker into a `<lic>` when needed
 * updated tests as necessary
Merge pull request #165 from nlbdev/schema-cleanup

A bunch of schema improvements, some bugfixes, ++