This reintroduces some old code in order to give the user the option to
switch between those two behaviours.
It also prevents the removal of all caps nodes (and thusly breaking the
new detection) when a rogue offline presence event comes in. This is
such an event in which a contact is falsely assumed as offline, if the
preferred resource of two or more same prioritised resources goes
offline. We are checking this by looking whether it is a
"resourceChanged"-event. This should probably be changed again, after
those offline presence updates have been fixed.