Static plugins: support loading static plugins from several engines

Static plugins: support loading static plugins from several engines

When loading dynamic plugins, we register them as loaded so that
we don't try to register their types several times if using several
engines. The same was not done for static plugins. This patch
will ensure that we follow the same logic also for static
plugins.

Task-number: QTBUG-36532
Change-Id: Icc1e089ae5d682c38b2d36bf4808f1c753c122a4
Reviewed-by: Simon Hausmann <simon.hausmann@digia.com>
Reviewed-by: Lars Knoll <lars.knoll@digia.com>