CFileItem: add m_iPosition property
tests: add unit tests to make sure m_iPosition is properly populated
sorting: add SortByUnsorted as a replacement for the original SORT_METHOD_UNSORTED (fixes #10252)
make use of the new CFileItem::m_iPosition instead of abusing m_iprogramCount
CFileItem: rename AddFront() to Insert()
use SortSpecialOnTop instead of CFileItem::Insert(item, 0)