QString only uses UTF-16, not UCS-4

QString only uses UTF-16, not UCS-4

This means that, when a string contains high-plane Unicode characters,
the length of a docstring and the corresponding QString will be
different: Qt will encode these characters using several 16bit
characters.

We have additionally to take into account QTBUG-25536, which implies
that sometimes qstring_to_ucs4(toqstr(s)) !=s. It is not clear whether
this bug can be a problem in other places.

Fixes bug #10443.
(cherry picked from commit 5d85a42bf0a182585b800a80817a6e1208ef7ec3)