🍒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: 5.27.1

(cherry picked from commit 4649594a)