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: {F7248196}

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