Both copying and drag-and-drop of images requires adding appropriate
mime data to the clipboard and drag handler respectively.
f365a38fa28f implemented support for copying modified images. To be able
to also support drag-and-drop of modified images without having to
duplicate code, we extract the relevant bits into a standalone helper
function.
As in different places the selection is determined either by Qt's
internal models, contextManager or a single document, and we don't
have access to the global selection in contextManager everywhere, we
standardize on a KFileItemList to pass the selection.
While at it, we slightly improve some comments.