ebiten: `AppendInputChars` should include a non-printable character

[hajimehoshi]

Now the result of AppendInputChars doesn't include a non-printable character (i.e., a character that returns false against unicode.IsPrint). For example, a zero-width non-joining space can be input from a Bolnagri keyboard directly [1]. We should not omit such characters.

As this is a breaking change, let's revisit this at v3.0.0.

[1] https://www.w3.org/TR/uievents-key/#h-clipboard-read