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>
QQuickWindow: let clearFocusObject actually clear the focus object

On iOS we clear the focus object when the virtual keyboard is
hidden (to stop blinking cursors etc). The current implementation
in QQuickWindow would then tell the root item to clear
focus. This was wrong, since setFocus is expected to be called
on the item you either want to gain, or release, focus (not the root
item). This sometimes caused items to end up in an incorrect
focus state.

With this patch we ensure that we call setFocus on the focus object.

Change-Id: I0c08fbc35f6695064b7c911466fdc62f4027385e
Reviewed-by: Shawn Rutledge <shawn.rutledge@digia.com>
Reviewed-by: Frederik Gladhorn <frederik.gladhorn@digia.com>