Added a function decodeString() to util.{h|cpp} that takes a QByteArray with raw...

it using a specified text codec. Except the input data is utf8, in which case this function detects
utf8 and takes appropriate measures to get a correctly decoded QString regardless of the specified coded.

I'd like to use this function for decoding incoming server messages, but we need to change things quite
a bit in the Server to do this correctly. See my comment in server.cpp.