Add option to additionally save screenshot to clipboard

Summary:
FEATURE: 390415
FIXED-IN: 19.12.0

Add an option to save the screenshot to the clipboard after it has been taken.

Test Plan:

Reviewers: Spectacle, VDG, davidre, ngraham

Reviewed By: Spectacle, VDG, davidre, ngraham

Subscribers: jborrero, muvori, davidre, ngraham, Spectacle

Tags: Spectacle

Differential Revision: https://phabricator.kde.org/D23162