Add auto mode in lightroom.

This mode is used when auto-importing tags in lightable or develop
processes in darkroom.
If no dt sidecar found, check for lr one.

At this stage we only import the tags.
Import lr develop process the first time we visit an image.
Revert "Add Lr cairo button for the import from Lr support."

This reverts commit 01861d50b9561f8a980247efcc8b0969c7fa3e5c.
Remove Lr button from darkroom.
Check that the XMP has really been produced by Lightroom.