Use the newly added clipboard for entire images to copy/paste layers
(we only create single-layer clipboard images, and use only the first
layer of any recieved image, the layers can be arbitrarily complex
though):

- change gimp_edit_copy,cut,paste() to return/take a GimpObject
  that can be a GimpImage or GimpBuffer
- cut/copy the whole layer if there is no selection
- always paste layers as new layers, not floating selections
- always paste news layers on top of the active layer, where
  we would attach a floating selection
- add enum GimpPasteType { FLOATING, FLOATING_INTO, NEW_LAYER }
- add GimpPasteType parameter to gimp_edit_paste() and handle all
  three cases there because there is now a lot of common code
  involved
- change all callers accordingly, use only legacy buffer pasting
  from the PDB for now