QMetaObject::indexfOfMethod returns the method corresponding to a
specific signature. In QML, we however only want any of the methods with
a given name (and do overload resolution at a later point).
For this usecase this patch introduces the internal
QMetaObject::firstMethod function.
Change-Id: Ie3820354edffb273c4cbe1399201a955ebe79344
Reviewed-by: Lars Knoll <lars.knoll@qt.io>