Simplified a bunch of the logic in the demo2 example
- Moved destroy mode to its own file.
- Moved error handling to its own file.
- Removed AppContext
- Implemented Widget for &App. The app state is small enough that it
doesn't matter here and we could just copy or clone the app state on
every frame, but for larger apps this can be a significant performance
improvement.
- Made the tabs stateful
- Made the term module just a collection of functions rather than a
struct.
- Changed to use color_eyre for error handling.
- Changed keyboard shortcuts and rearranged the bottom bar.
- Use strum for the tabs enum.
