beamer / TODO

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