VBAP panner UI tweaks:

* fix mouse-grab of sentinel
* make GUI more hemisphere like (circles at latitude)
* change alpha slightly to show signal overlap
take process lock when adding processors:

fixes possible crash if a processor modifies port-count

1. a processor is inserted and activated with processor-lock held
2. only after that the process_lock() is taken, configure_processors() is called which reconfigures-IO

BUT if the processor that is inserted changes the channel count AND audio is processed before IOs are reconfigured
 -> possible crash (invalid port-buffers)

To reproduce: Bus1 (2in, 3out), Bus2 (2in, 3out)
- add a send from Bus1 to Bus2,
- then add a processor to Bus1, just before the send which
  increases the channel-count to 4 -> occasional crash or assert.
fix processor -> reconfigure I/O || process concurrency

Add a ReaderLock to Route::process_output_buffers().

But process_output_buffers() is always called with processor-lock
held. To avoid deadlocks, a processor WriterLock must always imply
a process-lock (IFF reconfigure-I/O is called with _processor_lock).

Otherwise: e.g.
*  add_processor() -> takes processor-lock. set up and activate processor.
*  simult. audio-engine process, process-lock -> call process_output_buffers() -> wait for processor-lock
*  add_processor() continues -> calls reconfigure-io -> take process-lock -> deadlock.
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
fix delivery: check if panshell exists on session-load
delete 'big' 2d panner window, if panner changes to mono|stereo
allow to custom select panner-type for each delivery.
center VBAP UI crosshair
first stab at send+route panner link
add preference -- link-send-and-route-panner default
new MIDI binding map for Roland V-Studio 20, from Keith (Milner?)
fix compilation problem on OS X caused by a missing implementation of a virtual method
remove debug code