Expose EditableText.SetTextContents on Unix

[mirsella]

Closes #761.

This exposes AT-SPI EditableText for writable text inputs that handle string SetValue requests. SetTextContents forwards the new contents as Action::SetValue with ActionData::Value.

Numeric controls continue to use AT-SPI Value. The remaining EditableText methods return NotSupported because AccessKit does not have matching clipboard or range-editing actions.