The codebase now uses QStandardPaths everywhere. This ensures that there
will be no difference between how Akonadi sees default paths and how other
parts of the stack (especially those parts that use Frameworks) see them,
as they are all using the same method to look up the paths. This is
especially important on Windows where XDG_* environment variables don't
exist and are not honored by QStandardPaths. XdgBaseDirs still tried to
simulate XDG-like directory structure even on Windows, which caused
QStandardPaths-based code (like KConfig) to not find the correct config
files.