clipboard: Run data fetching in the correct thread

David Redondorequested to merge
work/davidre/clipboard-correct-thread into master

KSystemClipboard changed so it now runs in a separate thread. Because we override retrieveData of our QMimeData this runs in this new thread now. As a short term fix make it run in the correct thread again. Longer term we should port it to use data control directly without the intermediate layer of KSystemClipboard. CCBUG:515465

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