Add doxygen-extra.css, deprecate doxygen.css

* README: Say that doxygen.css is deprecated. Add description of
doxygen-extra.css.
* Makefile.am: Distribute doxygen-extra.css.
* build/doc-reference.am: Distribute doxygen-extra.css.
* skeletonmm/.gitignore: Ignore doxygen-extra.css.
* skeletonmm/doc/reference/Doxyfile.in: Use doxygen-extra.css instead
of doxygen.css.
* util/doxygen.css: Add comment, saying it's deprecated.
* util/doxygen-extra.css: New file. Equal to doxygen.css except for comments.
* util/mm-common-prepare.in: Copy doxygen-extra.css.
* util/mm-common-prepare.1.in: List doxygen-extra.css.
https://mail.gnome.org/archives/gtkmm-list/2014-August/msg00022.html