Filter function does not work with non-ASCII characters

[Eucyon]

To reproduce, press F4 and insert e.g. ä, ö or ü.

Thank you!

[fasterit]

The getch() we are using has no idea about how to process multi-byte input properly.
See https://github.com/ThomasDickey/dialog-snapshots/blob/7289c6063b40654f9818372cdbb9d17378ccc46a/ui_getc.c#L333 for an example of how to manage this better.
Code from dialog is LGPL v2.1 so compatible. Hint, hint.