* Get rid of different dconf trees for reading and writing again; the one single
  tree is /org/libreoffice/registry/.

* Trigger the special write-back mode when a file
  $SYSUSERCONFIG/libreoffice/dconfwrite is present; then, ignore the default
  user:!... entry in CONFIGURATION_LAYERS (i.e., don't use the
  UserInstallation's registrymodifications.xcu) and instead write changes back
  to the dconf tree.

Change-Id: I6b4d7251851da3fe7abbc9dcb080d57b278de96b