There are quite a few tools that might want to change certain values
based on consistent keyboard shortcuts. This commit allows tools to
hook up a "primary" and "secondary" slider with the base `Tool`
class, which can then handle updating those sliders with the common
shortcuts.
Note that any derived classes that want to override the `on_keydown`
function will manually need to call `Tool::on_keydown()`.
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 *