Add script to synchronize from Lr database.

These scripts can be used to synchronize rating, geo-tagging,
selected status and color labels from Lightroom.

This cannot be automatized as it needs access to the Lr catalog (SQLite database). So the
scripts here are helpers that can be run manually when needed to get valuable information
from Lightroom catalog to the darktable one.

I think merging into the main darktable repo will make those scritps more visible and will help people migrating.

Ok?