Add a config for isort (https://github.com/timothycrosley/isort)
Run isort -rc . and make import style consistent across files
Add a note about `isort` in CONTRIBUTING.md
Some noticeable points:
* `save_state` can check whether table is loaded or not. No need to
check it in `accept` and `reject`. Also `table_loaded` isn't required
anymore.
* Keep file formats list in alphabatic order, in `makeqrc.py`.
* Import `Track` and use `isinstance` to check whether object belongs to
it.
