Function `sequencer_refresh_sound_length_recursive()` can change strip
length, which can cause strips to overlap and break strip position.
This is prevented by ensuring right handle position is fixed.
Strip length should only change when user changes source file manually.
`SEQ_sound_update_for_new_sound_file()` function was added that will
update strips using specific sound and strip length is changed only if
strip has no offset.
Added docs to destinguish purpose of these functions.