AboutSystem: move authors and translators info to Credits.h

Signed-off-by: Axel Dörfler <axeld@pinc-software.de>
AboutSystem: use parens instead of dashes to indicate nicknames

Having dashes is confusing because some people have dashes in their names,
so using parens eliminates the confusion.

Signed-off-by: Axel Dörfler <axeld@pinc-software.de>
AboutSystem: minor coding style cleanup.