Excepting QBitmap and the QTextFormat heirarchy, which don't
have 100% value semantics (still pondering them).

In QPolygon(F), adding the move assignment operator disables
the implicitly-defined copy assignment operator, which therefore
have to be made user-defined. That doesn't change the ABI of
the class, since the base class QVector already had a user-defined
copy assignment operator.

Change-Id: I0b111c1d21cf47f559ada6357c982e3dc26aca68
Reviewed-by: Olivier Goffart (Woboq GmbH) <ogoffart@woboq.com>