Input method support for QQuickWidget

Make sure QGuiApplication::focusObject() returns the QQuickItem
that has focus, and forward input method events.

Task-number: QTBUG-42677
Change-Id: Ic07f93a2529c3dde67272b489bdc61fd25582a69
Reviewed-by: Laszlo Agocs <laszlo.agocs@theqtcompany.com>
Reviewed-by: Jørgen Lind <jorgen.lind@theqtcompany.com>