At the application level, support the existence of subdirectories inside
the original config and data paths for a specific version of the
software. For a new installation, create them, but for an existing
installation use the most recent available version, even if it's not the
current one (and even if it's not versioned at all). Any migration must
be done at the GUI level due to the amount of data that is potentially
being copied during that process.
Use std::filesystem wherever possible, replacing most uses of
std::string when the object is actually a path. This is the first stage
of refactoring, and does not make any changes to Application that affect
client code. Access to the new directory-handling class is implemented,
but is unused by any external code.
