klipper: store QImage and construct QPixmap only when necessary

The mime data from Wayland clipboard is not the exact same image, so two
image items are listed (first time restarting plasmashell). But after
images are saved to local, they have the same uuid again. So klipper
starts to become confused after the second time restarting plasmashell.

This is caused by QPixmap::from(QImage).toImage() erasing metadata in the
image, and Wayland clipboard is async, so another identical image is added
to clipboard.

BUG: 465225
BUG: 465326
BUG: 464828
BUG: 465603
FIXED-IN: 5.27


(cherry picked from commit 4649594a)