This makes things more consistent and allows us also to add more
search paths in the future if necessary.
Also, stop using g_module_build_path() because it's deprecated and
build the path manually. This obviously is not portable to Windows/Mac
or other exotic platforms, but portability is not important at this
point. PipeWire is also not portable beyond Linux & BSD.
This adds support for the system-wide locations for configuration and
data files, as defined by the XDG Base Directory Specification.
In addition, it adds two flag groups, CONFIGURATION and DATA, to the
base-dirs system, so that we don't have to hard-code the combinations
of flags everywhere.
