Update dconf / dconf-editor to 0.16.0

Many improvements, bug fixes and translation updates:
https://git.gnome.org/browse/dconf/tree/NEWS?id=0.16.0

Highlights:

This release more or less completes the rewriting of dconf-service.
Support for backends are now fully-realised (and we have a couple of
them as proof).  Many small bugs (some with serious symptoms) have been
fixed vs. the last release.

 - new icons: hicolor icons have been updated and a HighContrast set
   added

 - loosen some assertions on the order of messages being returned from
   the service.  D-Bus violates this order in the case that the service
   crashes and that's no reason to take the client with it as well.

 - add some backends to the dconf-service (that were 'considered' as per
   the last NEWS).  The first backend is a null backend that allows for
   a temporary database tied to the lifetime of the session, possibly
   useful for testing.

 - also add a new 'keyfile' backend for the service.  This is primarily
   intended to be used with NFS (where use of mmap is not safe)

 - add a mkdir() in the case of an empty home directory where
   ~/.config/dconf does not already exist

 - the service has been substantially rewritten
  - using gdbus-codegen
  - cache the file contents instead of read/modify/write each time
  - new DConfWriter abstraction allows considering support for backends
  - uses only POSIX IO (read and write syscalls) for database access
  - workaround mmap() bug on OpenBSD
Updated devel/dconf to 0.16.0
Build fixes