Positioner: Check if we need to defer applying positions during model conversion

When converting folderModelData, we should first check if we're still listing or not, and apply that to the m_deferApplyPositions.

If we defer, we quit early as usual. If not, we set the defer off, even when we do nothing: This allows any other methods to continue loading items as normal.

Previously this was not properly reset, causing desktop not loading new items after deleting any.

BUG: 516559 FIXED-IN: 6.6.2