Don't split UTF8 when handling accelerators (#983)

Marking up accelerator keys still assumed all characters were one byte
wide. Change this to accept multi-byte characters.
Warn on unsupported accelerator keys

Chars outside the basic latin alphabet don't actually work as
accelerator keys. Emit a warning when encountering them.
lang/de: Don't use 'ü' as accelerator

Taking over a player had the 'ü' key assigned as accelerator. Since that
doesn't work anyway, switch to a more conservative 'n'.