QmlDesigner: Add plugin directory to the exported directories

Change-Id: If230176bc7e96476e2f748d850a5a4c93b57cafa
Reviewed-by: Thomas Hartmann <Thomas.Hartmann@digia.com>
QmlDesigner: Export DesignDocument

Change-Id: I3aa1217c990ae36a43b269d8177fc5566b1c9dea
Reviewed-by: Thomas Hartmann <Thomas.Hartmann@digia.com>