Don't copy MaterialItem

Under gcc 10.2.1, I get the warning

/G/sw/ba/GUI/Model/Material/MaterialItem.cpp:56:1: warning: base class ‘class QObject’ should be explicitly initialized in the copy constructor [-Wextra]
   56 | MaterialItem::MaterialItem(const MaterialItem& other)

However, QObject should not be copied: https://stackoverflow.com/a/19092698/1017348