Support "system" properties, i.e., library versions, whether
language-detection, scm is supported.
Make these more generic (with display strings), so we can handle them
more automatic int mu-cmd-info.
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.
