Implement QTableWidget/QListWidget::mimeData()

For some reasons lost in history, those methods simply ignore
the list of items passed to them, and always return NULL.
Of course, that's totally broken. Fix that by doing the same thing
that the respective (hidden) models did when calling mimeData().

Surprisingly enough, QTreeWidget has a proper implementation.

Task-number: QTBUG-25900
Change-Id: I99ca7b4d4f87f096fd9ff0b0cb4e58282aa9c26e
Reviewed-by: Olivier Goffart <ogoffart@woboq.com>
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>
Reviewed-by: David Faure <david.faure@kdab.com>
Add QKeySequence::Backspace

Handle it in QWidgetTextControl. Add Meta(ctrl) + H
key sequence binding on Mac.

Task-number: QTBUG-32837
Change-Id: Ib1cbb2ce2e20350b4d8c72bd320dff27c1467f2b
Reviewed-by: Shawn Rutledge <shawn.rutledge@digia.com>
Reviewed-by: Liang Qi <liang.qi@theqtcompany.com>