Add option to copy screenshot to clipboard in background mode

Summary:
FEATURE: 385559
BUG: 393708
FIXED-IN: 19.04.0

Reviewers: Spectacle, ngraham

Reviewed By: Spectacle, ngraham

Subscribers: ngraham, davidre, Spectacle

Tags: Spectacle

Differential Revision: https://phabricator.kde.org/D19715