EpisodeWidget as a state machine

Experimenting with wrapping widgets and later on stacks into state machines.

good reads:

• https://hoverbear.org/2016/10/12/rust-state-machine-pattern/
• https://insanitybit.github.io/2016/05/30/beyond-memory-safety-with-types
• https://dev.to/mindflavor/lets-build-zork-using-rust-1opm
• https://yoric.github.io/post/rust-typestate/

Things left to do:

• Validate that the current states are correct.
• Make date_label a state machine(minor task, just got bored)
• Right tests to ensure the valid states won't brake
• Update Changelog.md
• Try to refactor whatever is possible
• Run clippy