When a selection exists, we are copying then pasting the selection
contents. In particular, with multi-layer selection, it means pasting a
merged result of the selected layers (like a sample merged but limited
to selected layers).

Yet when no selection exists, with a single layer selected, a cut in
particular would remove the layer fully, then a paste would copy it
elsewhere (in the same image or even on a different image). This was
still working, but not with multiple layers. This is now fixed and we
can now copy/cut then paste several layers (without merge), which is
sometimes a very practical way to move layers (sometimes simpler than
drag'n drop, especially between images).

As a consequence, the PDB function gimp_edit_paste() now also returns an
array of layers (not a single layer).