Change default Klipper behavior to accept images

Summary:
Change the default Klipper behavior in order for https://phabricator.kde.org/D19715 to work.

Klipper will save images to clipboard, but unless Ignore Images is unchecked in the options, it won't save them to clipboard history.

CCBUG: 393708

Reviewers: #plasma_workspaces, davidedmundson

Reviewed By: #plasma_workspaces, davidedmundson

Subscribers: ngraham, davidedmundson, plasma-devel, #plasma_workspaces

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D19852