fix deadlock when removing monitoring-section
ignore additional channels for AFL, PFL:

If the monitor-section has fewer-channels than the solo-listen point:
ignore additional channels.
update internal-send port-count when target port-count changes
add independent panner for internal (Aux) sends
fix output metering for Sends (Aux and External)
independent panning for external sends
misc panning related UI tweaks:

* connect 2D panner "edit" to big window
* disconnect 2D-panner GUI when it's visible but panner-type changes
* ignore mixer-strip level-meter context-menu for Aux-sends
ProcessorWindowProxy::processor_going_away takes care of deletion