QClipboard: Fix autotest fails

QClipboard: Fix autotest fails

tst_qclipboard.cpp still has Q_WS_WIN which must be clear away. After
clean up, auto test will fail under windows: When setMimeData() is
called, dataChanged() signal will be emited twice.

The solution for QTBUG-24184 has partially solved the problem, but it
still there. Make sure emitChanged() only called by QPlatformClipboard
will give our more control for this.

Task-number: QTBUG-24484
Change-Id: I23566c6d3b32828b6865234c311af3635fe9e299
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@nokia.com>
Reviewed-by: Samuel Rødal <samuel.rodal@nokia.com>