Since Calculator was converted to use the MPFR floating-point library, it has been conveyed under the GPLv3+ license (see the COPYING file) as that's the license of MPFR. However, I left the licenses of the individual files as GPLv2+ to make it possible to copy code from Calculator to other GNOME apps that have not upgraded to GPLv3+.

I recently realized it is super confusing to have the GPLv2+ license on individual files that use MPFR, so I've already upgraded the license on those files (mpfr.vapi, number.vala) as there is no practical way to use those files under the terms of GPLv2.

This patch bumps the GPL version on all other files. It's really optional as to whether we do this or not. At the time, I thought it best to leave it as GPLv2+ in order to facilitate more code copying; nowadays I'm inclined to reduce potential confusion by using GPLv3+ consistently, and to maximize the amount of GPLv3+ code in GNOME. MPFR's use of GPLv3+ was a good example of how one project's switch could induce others to switch; it's not so likely to happen for Calculator, but couldn't hurt.

Note that we can bump the GPL version unilaterally due to the "or (at your option) any later version" stipulation in the license headers; without this, we'd need permission from the copyright holders.