QmlDesigner.PropertyEditor: refactor QML file structure

All the "helper widgets" are now a plugin called HelperWidgets.
This allows reuse of these widgets from other places.

The QtQuick directory now really contains only the QtQuick specific
items.

Change-Id: I99dbbe43b25ee8631b6bb1effe993f951e19a8db
Reviewed-by: Thomas Hartmann <Thomas.Hartmann@digia.com>