This is still mcomix compatible with python3, not mcomix in python3.
We have a lot of works to do before adding new features.
Even this TODO is todo.

Code should be strictly correct:
- fix deprecation warning from gtk.
- avoid workaround for bug of dependences.
- ...

Code should be written in python3:
- using context manager if supported.
- using generator if supported.
- no `unicode`
- ...

Code should be simple and readable:
- using `[ for loop ]` instead of `map` and `filter` if `list` is needed.
- seprate big template into another file.
- ...

Code should be consistent:
- using single quote
- remove extra whitespaces
- ...