StringInput better unicode handling

[jvalkeal]

Non-english characters are not bound when using StringInput. For new view system this was done in #845 but was never added to older flow components. Should mimic what was done there.

There's still going to be issues with Grapheme Clusters which some asian characters are using and are traditionally difficult to support in terminals. There's an old issue #841 for this.

Also looking on which versions to backport this.