There should be no functional changes visible from this change, but this commit should make it easier to code tools which need to check on tweeakmode status, by making it easier to figure out which NLA Track contains the strip which owned the action being edited. (The strip is already saved, so this commit just adds the track alongside it). For now there is no version patch for this. The worst that happens is that an extra refresh is needed in the NLA editor to get these to show up.