beamer / TODO

 1) Add option to show covered stuff in a shaded way (instead of
    completely hidden).
 2) Use xcolor for better color management
 3) Some themes more appropriate for presentations of lectures.
 4) Explain in the user guide, how hyperref can be used to create
    non-linear talks.
 5) Make adding navigation buttons like a "back button" easy.
 6) Perhaps add an way of showing photos of the authors and the
    institutes in a convenient way.
 7) Put internal macros in beamer.cls into a namespace
 8) "Cleanup" beamer.cls
 9) Document some of the commands for creating headline and footlines.