Encrypt the account data files

[Christian-Stieber]

As it says. These files contain secrets, so they need to be encrypted.

The way this will work is that the user has to enter the password at the start of the application. Or maybe through some less secure way as well, not sure yet.

We'll just that one key for all files.

On Windows, we also have a CryptProtectData() function, and/or a "credentials manager". Look at whether these can be used.