QmlDesigner: Refactor currentModel() getters in QmlDesignerPlugin

Change-Id: I6d5085b591439cc2e79ca0f9872a1c0497c5f220
Reviewed-by: Tim Jenssen <tim.jenssen@digia.com>
QmlDesigner: Delete manipulator items

If manipulator items should not be deleted their parent must be set them
to zero.

Change-Id: Ic54c073175bb4a58fe4ebc304243a83bc7ea1f3c
Reviewed-by: Tim Jenssen <tim.jenssen@digia.com>