Contrarily to Q_OBJECTs, Q_GADGETs are not guaranteed to
descend from a Q_GADGET. Here, we ensure that if the first
superclass is a Q_GADGET, then the derived class will be
treated as one. This allows gaps in the Q_GADGET hierarchy
while preventing from trying to link to the inexistent
staticMetaObject if there's no such ancestor.
Change-Id: If10fb952e23655102a425bb18fe8babaf447a47f
Reviewed-by: Olivier Goffart <ogoffart@woboq.com>
Reviewed-by: Jędrzej Nowacki <jedrzej.nowacki@theqtcompany.com>
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