Event handling (#5)

- Refactor `event` handling (heavily inspired by [crates-tui](https://github.com/ratatui/crates-tui/) via [Tui with Terminal and EventHandler](https://ratatui.rs/recipes/apps/terminal-and-event-handler/))
- Refactor widget structure
- Disable `nixos-unstable` temporarily
- Add `.rustfmt.toml`

src/widgets.rs

@@ -0,0 +1,5 @@
+pub mod countdown;
+pub mod footer;
+pub mod header;
+pub mod pomodoro;
+pub mod timer;