Save image to clipboard if --nonotify is used

Summary: BUG: 411263

Test Plan: run spectacle with --nonotify

Reviewers: #spectacle, davidre, ngraham

Reviewed By: #spectacle, davidre, ngraham

Subscribers: ngraham, #spectacle

Tags: #spectacle

Differential Revision: https://phabricator.kde.org/D23723