Tweaks to the API documentation

  * Update the Doxyfile for 1.7.6.1. Doxywizard reformatted the comments; that's why this change is so big
  * Disable graph inheritance trees; use "Inherits/Inherited by" lists instead (Doxygen has this on by default; I don't think it was intentional)
  * Shorten two subgroup names; helps with formatting in Doxygen 1.8 and looks cleaner
  * Moves the page footer ("The Haiku Book pre-R1...") be in a <footer> tag, and add a CSS rule for this tag
  * Disable XML output, no one is using it
  * Disable Microsoft IDL parsing, speeds up Doxygen a bit
Turn class diagrams back on