It gets stored in new hb_audio_config_t.in.name field.

It is available in the title dict read through hb_title_to_dict() or
hb_title_to_json() in AudioList[].Name.

When a job is initialized with hb_preset_job_init or
hb_preset_job_init_json(), output audio tracks are initialized with the
source track name.

Also adds output track name initialization to LinGui.