Change leader in group

[wpferguson]

Using lua scripts, we can solve both of these.

We could add a post import film filter to go through the film roll and make the jpeg the group leader
We could add a shortcut cycle though grouped images, selecting the next one and wrapping around at the end. We could also add another shortcut to make the selected image the group leader.

See darktable-org/darktable#9503