frontends:qt: ensure the packet list is displaying packets in original order

[fvdpol]

Since the analysis stack only sends complete packets to the frontend's visualisation queue when complete, this occasionally results in packets displayed out-of-order in the packet list.

This PR adds a check to safeguard the sequence of packets added to the QTreeWidget and in case these are out-of-sequence ensures the QTreeWidget gets sorted in sequence number.

expanding the transfer at packet 1:

This fixes #90