QmlDesigner.NodeMetainfo: adding defaultPropertyIsComponent()

This can be used to check if reparenting makes sense.

Change-Id: Ide1146841be76d8979dc452954fc6965ecdfef3d
Reviewed-by: Marco Bubke <marco.bubke@digia.com>
QmlDesigner.FormEditor: using FormEditorItem::isContainer in MoveTool

Change-Id: Ie30c0409cb2288c8c2cf649a92e7b54aa9481cb2
Reviewed-by: Marco Bubke <marco.bubke@digia.com>