This removes the need to edit the copyright year before generating
the documentation (every year, in several files) and adds some
technical information (doxygen generation date, doxygen version,
and FLTK Git revision) in both HTML and PDF docs.
- auto-generate copyright year (current year) used in several places
- include FLTK Git revision in HTML and PDF docs
- include generation date and doxygen version
- replace special html footer which didn't work well with default footer
