SPDX standardizes how source code conveys its copyright and licensing
information. See https://spdx.github.io/spdx-spec/1-rationale/ . SPDX
tags are adopted in many large projects, including things like the Linux
kernel.
ControllerEmu::Control instances have a unique_ptr<ControlReference>
member, which is passed either an InputReference or OutputReference.
Without this virtual destructor, deleting a derived class through a
pointer to the base class is undefined behavior.
Better separation of concerns. Relegates `ControllerInterface` to
enumerating input controls, and the new `ControlReference` deals with
combining inputs and configuration expression parsing.
