Don't assume unmapped keycodes outside the extended ASCII range are unprintable

This hack fixes #5871 (closed).

arg.keysym.sym is 1105 for the character in question. The regular (Latin) letter keys still produce their Latin syms when changing the keyboard layout to Russian so I'm not sure what the deal is.

std::isprint appears to be locale sensitive (because we can never have nice things), both it and std::iscntrl treat ё as a control character on my machine.