app, menus, pdb: new "Paste as Single Layer( in Place)?" actions.

When the clipboard contains raw image data or single layers, it's the same as
the normal "Paste" (and "Paste In Place" respectively). These actions are useful
if you want to copy a bunch of layers and paste them "merged" into a single
layers (since now the copy-paste of multiple layers will create multiple
layers).
It is somehow similar to the "Copy Visible" action except that it works on
selected layers only and work at paste time, making the action more versatile.