Made the following improvements pertaining to the ACL2-Doc Emacs
browser (some in acl2-devel, some in acl2-books):

  Files foo.acl2-doc will come up in ACL2-Doc mode, as requested by
  Warren (convenient for saving a file of bookmarks).

  Modified books/xdoc/display.lisp to print {IMAGE} or {ICON} for a
  missing image or icon (formerly these were just ignored when
  printing text).  Users should get the hint that these topics are
  best viewed on the web.

  A new ACL2-Doc command, H, displays the history in a special buffer.
  This can be convenient for revisiting topics.  Thanks to Nathan
  Wetzler for suggesting this improvement.

  A "type h for help" message now occurs at the bottom of the screen
  when starting up ACL2-Doc.

  A new ACL2-Doc command, D, forces a new download from the web of the
  acl2+books combined manual.

  The acl2-doc buffer is now in the directory of the file from which
  it was derived, as suggested by Warren.

  A new ACL2-Doc command, <Space>, is the same as Control-v (scroll
  up).

  Source information -- a book name or "ACL2 Sources" -- is now shown
  near the top of the page, when browsing the acl2+books combined
  manual with ACL2-Doc.

  If the combined manual exists in gzipped form only, say because it
  was manually downloaded from the web but not gunzipped, then at
  startup ACL2-Doc will offer to gunzip it.

  ACL2-Doc now displays which manual -- "acl2+books combined manual"
  or "ACL2 User's manual" -- at the bottom of the screen along with
  the topic name.

  A new command, <TAB>, tabs through the links on the displayed page.

  A failed attempts to download the combined manual may result in an
  improved error message.

  Fixed top page of the acl2+books combined manual to match what
  appears on the web, instead of being generic.


git-svn-id: http://acl2-devel.googlecode.com/svn/trunk@1189 5d84a466-a78b-0930-a311-59b22171a541