Fix composite type property support

Fix composite type property support

The metatype changes caused a regression in composite type properties
for which there was no autotest.  In particular, declarative previously
used the transitional QMetaType::registerType() function, whose
implementation specifies a zero-size, which means that accessing a
property of that type via QObject::property would fail.

This commit adds some unit tests to track the issue, and also fixes the
implementation of QQmlEnginePrivate::registerCompositeType to use the
updated metatype registration functions.

Task-number: QTBUG-25697
Change-Id: Ic4abc1711aee8cf4a50a2ef1769465ef73f5d022
Reviewed-by: Jędrzej Nowacki <jedrzej.nowacki@nokia.com>