Todo-indicator

Added

- This changelog! It's never too late, right?!

Changed

- Show the list's actual filename in "Edit" menu item instead of hard-coded
`todo.txt`. ([PR 6](https://github.com/keithfancher/Todo-Indicator/pull/6).)
- Update module structure for proper packaging and installation.
- Update build system and package metadata.
- Documentation updates: bring things into the current century, mostly.

Added

- Python 3 support. ([PR 5](https://github.com/keithfancher/Todo-Indicator/pull/5).)

Fixed

- A few under-the-hood updates to how we import our dependencies. Un-break
things in more modern distros.