wchar_t / Q3DSChar mixing in DocumentEditor

This code is probably suspicious since CString::c_str() is defined as a cast to wchar_t

StaticAssert<sizeof(wchar_t) == sizeof(char16_t)>::valid_expression();

const wstring &theSubsetName = Q3DStudio::CString(theBuffer->m_Subsets[subsetIdx].m_Name.c_str()).c_str();