Prevent users installing to an existing directory even if they acknowledge the danger

[lazka]

If installing to an existing install it shows:

But if installing to an existing random directory it shows:

While there is a warning, I doubt there is a valid reason for anyone to click "Yes" there. The chance of the user not reading the message and a future uninstall deleting unrelated files is just too big.

The qt installer doesn't have a setting to make this stricter sadly: https://github.com/qtproject/installer-framework/blob/047bb926705b8fa290d2a276ab0869b2afaa96f9/src/libs/installer/packagemanagercore.cpp#L1134

But we could look into if we can abort the installer after that even if the user selected yes.