Complete `App::draw()` and `handle_events()` implementation and test for simple task creation

[abstralexis]

Make the app draw the four boxes for the task creation to the screen and then handle switching between them for text entry, then handle submission into an arbitrary .toml file.

[abstralexis]

I consider the closing of #2 with commit fe13a2c to have made an amount of progress towards this - I believe there will need to be a significant restructuring of the code to ensure that the use of tui-input can be implemented nicely and make it repeatable. Maybe it is worth making our own wrapper widget?