[Klipper] Always restore the last clipbboard item

Summary:
D19852 intrdouced x-kde-force-image-copy hint for programs to specify that
images should be set to the clipboard even if should not be included in the
history. But when the program was closed the clipboard was restored to the first
item in history in which the image was not included. To fix this introduce a new
class member which holds the last clipboard item and restore it.
Credit goes to David Edumndson
Test Plan:
Try it with spectacle
Reviewers:
@ngraham, @davidedmundson, #plasma
Subscribers: