Add a lightable/darkroom mode to Lr import.

Typically tags are imported in lightable when importing a file or folder
and development processes when entering the first time an image.
Get image width/height from Lr xmp.

This is better as we do not need to have the dev->image_storage fully
intialized. We can then use the import routine sooner in the process.
Do not try to free a null pointer.
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.