Add HasText::bindText

[mshabarov]

What is needed:

• Add default void bindText(Signal<String> textSignal) to HasText that delegates to getElement().bindText(textSignal).
• Same contract as for bind text in Element.
• Do not change Element: it already has bindText including lifecycle handling via TextBindingFeature and ElementEffect.bind.
• Rely on existing guards so setText throws during an active binding (enforced at Element).
• Keep memory overhead zero for components without bindings (no new fields; only a default delegating method).
• Binary/source compatible for all existing HasText implementors.