Calculator now uses the KeypadValue class instead of double in
its internal calculations. By not constantly converting to
double back-and-forth, we do not use precision simply by, for
example, negating a number. This fixes#7484.
SPDX License Identifiers are a more compact / standardized
way of representing file license information.
See: https://spdx.dev/resources/use/#identifiers
This was done with the `ambr` search and replace tool.
ambr --no-parent-ignore --key-from-file --rep-from-file key.txt rep.txt *
