Enable build of a reduced Doxygen version for compressed Qt help file. Currently a doxygen build is about 2 GB, certainly too much for any file distribution. Thus `make doxyqt` now creates a complete Doxygen build of about 70 MB without dot graphics and Javascript search tools. * doc/doxyhtml/Doxyfile.in: Update year. Build Javascipt search engine by default HTML generation. * doc/doxyhtml/DoxyfileQt.patch: A patch for Doxyfile.in to make the reduced build for compressed Qt help files. * doc/doxyhtml/module.mk: Add target `doxyqt` for reduced build in subfolder `doxyhtml/doxyqt`. Add new files to build system.

# The following lines are used to create a minimal version to be used
# as documentation within the GNU Octave GUI widget Qt Assistant.

HTML_OUTPUT            = doxyhtml/doxyqt
HAVE_DOT               = NO
GENERATE_TREEVIEW      = NO
SEARCHENGINE           = NO