Cooy and paste history from one image to another clears source image's history
Consider two images in the Lighttable A and B.
If adjustments of A are made in the Darkroom, and then in the Lighttable, one copies the history from A with CTRL+C and then pastes them on image B with CTRL+V,
the history is not applied to image B. Instead, image A's history is deleted.
Steps to reproduce:
1. Open some images in the Lighttable
2. Make adjustments to an image in the Darkroom
3. In the lightroom, copy the changes from the image with CTRL+C
4. Paste the changes to another image with CTRL+V
darkroom: unref current developped image when leaving the darkroom.