Blender Git Loki

NLA: Store track that the strip being tweaked comes from

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.