Refactor determining mime data for selection

Summary:
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.

Test Plan:
Make sure that copying both unmodified and modified images of various
filetypes works as before and in all modes, in particular Browse,
View and Compare (for details, see test plan of
f365a38fa28f).

Reviewers: Gwenview, muhlenpfordt

Reviewed By: Gwenview, muhlenpfordt

Differential Revision: https://phabricator.kde.org/D13248