Add methods update_labels, clear_labels which update or clear the labels for a
message in the store, and update the cache with the overall counts of labels.
Add a LabelsCache to keep track of the counts and labels_map() to retrieve that
map.
Only include xapian.h in one place, so we can have consistent options.
With that in place, we can enable C++ move semantics.
We don't do anything with that yet, but we check in the meson.build file
to see if we have the required xapian version.
Xapian supports an "ngrams" option to help with languages/scripts
without explicit wordbreaks, such as Chinese / Japanese / Korean.
Add some plumbing for supporting this in mu as well. Experimental for
now.
The default was 250000 but that led to problems on some systems with
limited memory, esp. since mu's indexing does quite a bit more than in
the olden days (e.g. html mail).
Fixes#2529.
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.
