Properly incorporate Doxygen into the build system * Doxyfile.in: Rename from Doxyfile and move to doc/doxyhtml directory. Templatise properly with autoconf macros. Configuration tweaks: expand DEFUN macros, show namespaces, document the GUI sources, don't strip Doxygen comments from the source, allow dot to create larger graphs. Also add a a tagline and the Octave logo. * doc/doxyhtml/README: New file, grafted from the dev branch. * doc/doxyhtml/Makefile.am: New file, handles doxyhtml target. * Makefile.am: Add doxyhtml target. * configure.ac: Add doxyhtml files to AC_CONFIG_FILES