unstable/text-input: Add hint on how to handle uncommitted text on unfocus

Different IMEs have different expectations on what should happen if focus is moved between text widgets or moving the cursor when text is not committed.

In Korean it is expected that any pending word is submitted, whereas in Chinese or Japanese languages the expected user behaviour is that pending text is discarded.

This has to be delivered as a hint to the client ahead of time as when focus moves, we might not have an input object to send commits to.