0032546: Data Exchange, XCAF - mark material as defined XCAFDoc_VisMaterialPBR::IsDef...

0032546: Data Exchange, XCAF - mark material as defined XCAFDoc_VisMaterialPBR::IsDefined=true within default constructor

XCAFDoc_VisMaterialCommon and XCAFDoc_VisMaterialPBR default constructors now set IsDefined property to TRUE.
So that it is required to set IsDefined=false explicitly to create an undefined material,
which is done by XCAFDoc_VisMaterial constructor.

Existing application code shouldn't require any modifications in most cases
(IsDefined=true required before to setup material could be now removed).