These aren't necessarily cheap to copy, since a control qualifier will
have around 3 std::strings inside of it, so passing by value can churn
allocations a little bit.
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.
-add a way to reset their value (from the mappings UI)
-fix "memory leak" where they would never be cleaned,
one would be created every time you wrote a character after a "$"
-fix ability to create variables with an empty string by just writing "$" (+added error for it)
-Add $ operator to the UI operators list, to expose this functionality even more
This clashes with X11's preprocessor define named Success (because using
non-prefixed lowercase identifiers in C was apparently a fantastic idea
at some point), causing compilation errors.
Better separation of concerns. Relegates `ControllerInterface` to
enumerating input controls, and the new `ControlReference` deals with
combining inputs and configuration expression parsing.
