Fix hydra-tui forms to only submit valid values

[ch1bo]

Also shows focus, invalid values and the maximum amount available.

Fixes issue #224

Before:

After:

[abailly-iohk]

It's a bit sad we don't have more tests coverage on the TUI, but functional tests are expensive. If we keep adding features here, I would argue to start refactoring towards better testability, perhaps abstracting away the Terminal-based infrastructure and have a separate testable logic layer that only deals with updating some custom State and reacting to users' Actions ?

[github-actions]

Unit Test Results

    6 files  ±0    71 suites  ±0   4m 45s ⏱️ -17s
203 tests ±0  201 ✔️ ±0  2 💤 ±0  0 ❌ ±0 

Results for commit 69d3ec3. ± Comparison against base commit 601b759.

♻️ This comment has been updated with latest results.

[ch1bo]

For the State updating part that might actually be working. IIRC the EventM or Next brick data types were impossible to unwrap, but we could factor out another, more introspectable DSL than continue :: s -> EventM (Next s). As for the rendering, we needed the full terminal hijack (hacked) test suite, which we have right now.