Two additional bugs found after the initial xref cleanup:
1. Undo after unlink did not restore PLC variables (PLC_TYPE,
PLC_ADDRESS, etc.) because PlcLinkWidget::on_m_unlink_pb_clicked()
never stored the current group index in the LinkElementCommand.
makeLink() only populates PLC fields when group_idx >= 0, so
the slave was re-linked but appeared empty. Fixed by reading the
group index from the master via groupIndexForElement() and calling
setGroupIndex() before unlinking.
2. m_update_slave_Xref_connection was only cleared inside the
if(m_slave_Xref_item) block in the updateXref() cleanup path.
For AlignHCenter (Text field) position, no m_slave_Xref_item
is ever created — the connections are stored in
m_update_slave_Xref_connection but never cleaned up on unlink.
On re-link, the old stale entries prevented new connections from
being established. Fixed by clearing the list unconditionally
in the cleanup path.
Kellermorph