QVariant should support converting short to/from unsigned short etc.

The code seems to try to support conversions, but the docs http://doc.qt.io/qt-5/qvariant.html#canConvert clearly state that these conversions are not automatically supported.

Qt 5.5 appeeared to work, because it contained a bug that produced false-positives with canConvert(), among them allowing short -> ushort and QObject* -> ushort. This bug was fixed by https://codereview.qt-project.org/145198

Decide whether to allow these conversion and then implement and document them.