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