Add support for char16_t, char32_t and std::nullptr_t in QDebug

This is required before we can add support for those three types in
QVariant.

This commit changes the output format for QChar when it falls outside
the printable ASCII range. In the future, it might be nice to use the
pretty-printing of control characters like QtTest and QJsonDocument.

Change-Id: I4d942da8d11f83de9c1b485ea6ca804fe1622602
Reviewed-by: Jędrzej Nowacki <jedrzej.nowacki@theqtcompany.com>