First phase to true descriptors

This moves all convenience constructors to use frozen ReactDescriptors.
BeforeInputEventPlugin: Support Opera <= 12

Add support for Opera <= 12 (Presto) in `BeforeInputEventPlugin`.

It turns out that Opera 12 has a `TextEvent` in `window`, but doesn't actually fire any input events. Even `input` apparently doesn't fire. Fall back to keypress handling in this case.