add debug-message to track down missing Sources
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.
fix utter confusion about session _path in new sessions.

Yikes!
make backtrace symbols work again inside gtk2_ardour (-rdynamic is a link-time flag, not compile-time
Fix egregious logic bug in FileSource::removable() and introduce FileSource::is_stub() to hide logic for deciding if a source (file) is a stub
add FileSource::is_stub() declaration
when cleaning up sources, do not remove "stub" sources from the source list, even though they do not exist on disk yet; remove some debug output
equivalent change to realpath() error return as was done in master rev 60a9213035d3c
merge with master, with minor conflict fixes