Add non-templated versions of get[Static]ObjectField()

Ideally the getStaticObjectField() and getObjectField() methods, that
take a signature, should never have been templated. Removing the
templated versions now will cause ABI breakage, so the only solution
is to add new non-templated versions.

Change-Id: I2f6e9f259b7bc511ef5bcaeb8a2077b0f267b8c8
Reviewed-by: BogDan Vatra <bogdan@kde.org>