Implement focus change for Windows Input context.

Keep track of the focus object in the input context ensuring
that events on canceling are sent to the right object.

Task-number: QTBUG-40402
Change-Id: I79820db94d97e21b47abc8fe2bae6fa012d31236
Reviewed-by: Liang Qi <liang.qi@digia.com>
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>