Unify how image export is reported

Summary: Use the same control flow structure for `imageCopied` as `imageSaved` and `imageSavedAndCopied`.

Test Plan: Works as before.

Reviewers: #spectacle, aprcela, ngraham

Reviewed By: #spectacle, ngraham

Tags: #spectacle

Differential Revision: https://phabricator.kde.org/D23467