Fix Component.onCompleted emission and bindings for composite types

When instantiating the composite type with a sub QQmlObjectCreator,
make sure to link the componentAttached objects back into the parent
creator. Similarly we need to collect the bindings the sub-creator
created, as finalize() is only called on the top-level creator.

Fixes remaining failures in tst_qqmllanguage::onCompleted

Change-Id: If79b31ee94000aeec280284200df8ee97e57fd03
Reviewed-by: Lars Knoll <lars.knoll@digia.com>