The old struct config_param remains only for top-level string options.

Replaces the dirty code in config/ConfigFile.cxx.

The new class Path only holds a string pointer without being
responsible for allocation/deallocation.  The FileSystem.hxx library
accepts Path arguments instead of AllocatedPath, to avoid forcing
callers to allocate another string object.

Prepare to migrate away from GLib.  Currently, we're still using GLib
as a backend.

Use only ConfigData.hxx in plugin sources to reduce header
dependencies.

Replaces GLib's GError.

Look up top-level config options by enum (= integer), not by name
string.