Allow loading sessions with missing external files #7067

Allow loading sessions with missing external files #7067

The issue was introduced in dbf86a495b, forcing must_exist = true
for *all* files that have a absolute _origin, including stub
SilentFileSources.

If an external file is no longer available and a user chooses to
"ignore, skip" an SilentFile is created instead of the actual source,
The SilentFileSource has the same XML state which includes _origin.

ARDOUR::SilentFileSource::SilentFileSource () c'tor calls
AudioFileSource::AudioFileSource (,.., must_exit = false)
but since the file has an absolute _origin, the AudioFileSource c'tor sets
must_exist = true; throws and the session cannot be loaded.