update available panners when panshell changes

Fixes issue when adding splitting plugin
panner UI: check URI and #of channels on re-configuration
fix stereo-panner GUI edge case (panshel changes, send|listen)
don't add a [shared] panner for the sends to monitoing section

fixes issue with 'stuck' mono panners (when using a stereo
monitoring section and auto-connect): the send to monitor
section is added after the main panner. At this point the
channel-count is different, but the send shared panner() with
the main route.

Here: mono-panner on track, stereo-panner; and they shared control
settings. since stereo-panner's width is 1, the position cannot be
changed.