Replace Qt streams in the Wizard code

On non-Windows system we use the libunshield, and its worker uses Qt streams with legacy encodings:

mUnshield->setIniCodec(QTextCodec::codecForName("windows-1250"));

The problem is that Qt6 does not support legacy encodings, so we will need to use a different approach to encode strings (for example, use ICU or our ESM encoder).

Also it may be better just to drop the Retail CD support and use only Existing installation method. Using a retail CD on Linux and Mac OS is a very niche case, which can be handled via WINE.