Previously there was only a PostDisconnect() signal.
When a port connection is made directly e.g. recorder_ui.cc
r->input()->audio(..)->connect(...), or when connecting
using an external jack control app, Ardour::IO is not informed.
no IOChange::ConnectionsChanged signal is emitted and
Track::set_align_choice_from_io() is not called. This leads to
incorrec capture alignment.
To minimize signal emission the static global PostDisconnect
is changed into a per port ConnectedOrDisconnected signal.
IOs subscribes to signals from owned ports only.
3f6bf2c451