Serialize QPicture using the latest QDataStream version.

The QDataStream version with which QPicture was previously serialized
with was hard-coded to 11 (Qt 4.5). This prevents features of
serializable types used by QPicture from being serialized if they
were added after Qt 4.5; namely features of QFont like
HintingPreference.

[ChangeLog][QtGui][QPicture] QPicture now serializes its data properly
by also accounting for QDataStream versions greater than Qt 4.5.

Task-number: QTBUG-20578

Change-Id: Idd27682b187f4d4a3695c52bbf68e84853c024a8
Reviewed-by: Gunnar Sletta <gunnar.sletta@jollamobile.com>
Reviewed-by: Eskil Abrahamsen Blomfeldt <eskil.abrahamsen-blomfeldt@digia.com>