Improve ComponentBase and Container::Tab Focusable implementations (#341)

- Provide better defaults for ComponentBase `Focusable()` and
  `ActiveChild()` methods. This resolves:
  https://github.com/ArthurSonzogni/FTXUI/issues/335

- Implement `Container::Tab` 's  `Focusable()` methods. This prevents
  the users to navigate into a tab with no interactivity.

CHANGELOG.md

@@ -27,6 +27,9 @@ Element gaugeDirection(float ratio, GaugeDirection);
 ### Component
 - Support SIGTSTP. (ctrl+z).
 - Support task posting. `ScreenInteractive::Post(Task)`.
+- **bugfix** Container::Tab implements `Focusable()`.
+- **bugfix** Improved default implementations of ComponentBase `Focusable()` and
+  `ActiveChild()` methods.
 
 2.0.0
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