No need for "pimpl" in xapian-db; keep it simple.
Not complete, but a first big stab converting users of Mu::Error and various g_warning & friends, format to the new libfmt-based APIs.
XapianDb is a fairly thing wrapper around Xapian, which handles locking, exception handling and some tracking. On top of that, Config add a configuration database for type / introspectable configuration info.
