feat(widget): initial implementation of stateful rendering

[a-kenji]

Opened this branch, because I don't have permissions to push to #82

• fix stateful widget example
• simplify spawn function
• gate input handling behind unstable feature flag
• implement an async version
• implement viewport Implement ViewPort #14
• benchmark stateful version
• add tests for input handling, with different terminal variants
• write documentation
• add stateful sync/async examples to test builds
• optionally? add tracing crate for input handling events, to make it less of a black box
• carry reference to the child process e.g. a child killer
• (optional) buffered event handling
• add a timeout to the parser thread (when it gets sizes of 0).

[lthoerner]

What is preventing you from merging #82 ?

[a-kenji]

It is far from being done.

[lthoerner]

It is far from being done.

Oh, I misread your original post.

[a-kenji]

I think the stateful Widget is too complicated to reason about currently, while the Input handler is not refined yet.

So I will go with a different Interface for now.

The initial direction is in #133