... when there's a floating selection

Layer-dialog interaction is restricted while the image has a
floating selection, which often causes confusion.  Highlight the
three layers-dialog buttons that "finalize" a floating selection --
the "new layer" button, the "anchor layer" button, and the "delete
layer" button -- to indicate that these buttons are used to finish
the paste operation.  The "new" and "anchor" buttons use a green
highlight color, while the "delete" button uses a red highlight
color.