ClipboardManager didn't have any APIs to clear clipboard data before
API level 28.
As a workaround an empty Intent with MIMETYPE_UNKNOWN is created and
inserted into the clipboard for lower API levels.
This makes the QApplication::clipboard()->clear() method work more or
less as expected.
This allows to unblacklist tst_QPlainTextEdit::copyAvailable().
Task-number: QTBUG-87423
Task-number: QTBUG-89402
Pick-to: 6.3 6.2
Change-Id: I454376199cf3b8eed0fa2ecf2f85b87f40892280
Reviewed-by: Assam Boudjelthia <assam.boudjelthia@qt.io>
30830e0cba