replace usage of ExternalProperty

* in an ItemWithMaterial, store only the identifier of the used material
* hold the identifier as QString, not within an ExternalProperty
* modify the ItemWithMaterial/Material property to type "QString" instead of ExternalProperty
* adapt all methods which used the ExternalProperty to use the material identifier or a ptr to a MaterialItem
* adapt the XML reading/writing
* add legacy code to read old XML files
* adapt material selection via SessionModel's CustomEditors