Update copyright year
More or less a follow-up of !18 (merged).
By updating the copyright year in the about dialog, I realized that there was also an occurrence of 2020
in configure.ac.in
. So I made the two files consistent, @matt @lastonestanding do you have any objections? (The capital C in configure.ac.in
is to match the FSF copyright display when you do ./configure --version
).