fix up some confusion with filesources' _origin and _file_is_new members. if _origin...

fix up some confusion with filesources' _origin and _file_is_new members. if _origin is set, it means that the file is "external" to the session (aka "embedded") and for some purposes this is more significant than _file_is_new. rename SourceFactory::createReadable() to ::createExternal() to more clearly indicate its purpose; remove never-supplied "origin" argument from SourceFactor::createWritable(). Fixes problems caused by 864ce8f0