Resaving a document after adding a cel between existing cells causes the frame ordering to be incorrect upon reloading.

[illegalinstruction]

To repro:

1. Create a new, empty animation.
2. Add a cel to frame 1. (The problem is easiest to see if a '1' is drawn into it.)
3. Add a cel to frame 5. (Draw a '5' into it.)
4. Add a cel to frame 9. (Draw a '9' into it.)
5. Saved the document, then olen the saved version.
6. Add cels to frames 3 and 7.
7. Save again.
8. Reload the document, and look at the timeline.

Notice that the frames are now out of order, and the cel containing '9' is no longer in the ninth frame.

Suspected cause:

This seems to have something to do with the fact that internally, at least during serialization/deserialization, the app stores frame data in a 'dict', which isn't guaranteed to be ordered, and more often than not, the index each frame gets in the dict is based on what order it was created in, anyway.