Assign copyright for modifications to GC3.
Cosmetic changes.
Make `gc3.sty` independent from the beamer style.

In other words, `gc3.sty` no longer loads the beamer style `gc3`, so we
can --theoretically-- use it separately.
Fix header in `gc3.sty`
gc3.sty: Allow specifying an optional color for `\HL`.

Also, use `uzh@orange` as default highlighting color instead of
`RedViolet`.
Showcase features from `gc3.sty` in the example slide set.