Setting subpresentation to referenced material crashes

It is possible to set a subpresentation to a referenced material's image properties (IBL override and various lightmaps) via context menu or inspector, but doing so causes a crash. Dragging subpresentations to reference materials is blocked, so there is an inconsistency.

Custom materials work otherwise similarly, except they do not crash when setting the subprentation.

The functionality should be consistent and not crash. I suggest blocking all subpresentation setting on reference and custom materials in 2.1. In 2.2 material system overhaul will need changing how this functions, anyway (see QT3DS-2199).

Additionally, when setting a sub-presentation to a ref material, a check box should appears in the ChooseImagePropertyDlg to allow the user to change the material to a Standard material so that the sub-presentation setting only affects this material instance.