Add "gboolean merge_active_group" to gimp_image_merge_visible_layers()
and pass FALSE from the PDB wrapper so plug-in invoked layer merging
always acts on the toplevel container as it did in 2.6. Do the same
when opening an image as layer.

Also, added a "Merge within active group only" toggle to the merge
layers dialog and pass it down to the core, but default to TRUE here
so.