Rewrite Paste support to account for layers

There are now two supported Paste operations:

- Paste as new layer
- Paste as new image

"Paste as new layer" supports nearly all the same features as "Paste as
new image" - e.g. 32bpp data and file paths are handled just fine
(including multiple file copies from Explorer; each file will be loaded
as a new layer).

The one exception is URL paths.  This is not currently supported for
"Paste as new layer", but it's on my to-do list.