individual: Change an assertion into a warning and relax its conditions

This should fix a crash (probably) caused by a Persona which is in the
process of being moved between two Individuals emitting a property
change notification while both Individuals are still connected to its
‘notify’ signal.

This is only ‘probably’ because I can’t reproduce the problem.

See: https://bugzilla.redhat.com/show_bug.cgi?id=1031252
individual: Clean up how Personas are transferred between Individuals

Slightly improve it so that (theoretically) it’s now impossible for two
Individuals to be connected to the same Persona simultaneously. Now, the
old Individual will be automatically disconnected when the new
Individual is connected.