Enable usage in enviroments without C drive letter

[MagicalDrizzle]

The C drive letter is not required to use Windows. Using %USERPROFILE% is a much better idea than hardcoding the system drive.
This also enable portability of the data folder simply by redefining the env variable in a batch file.

[MagicalDrizzle]

That's my main proposal, and this is my second one which may or may not be a good idea since it only deals with portability.
If you override %USERPROFILE% to the current folder make the program portable, EmuGUI will fail to launch due to the Documents parent folder not existing.
There are two workarounds:

• You can simply create the folder yourself
• Or you can replace os.mkdir with pathlib.Path.mkdir, which has the option to create all parent folders if they don't exist.

Feel free to only merge the first commit if you don't think all this is worth it.

[Tech-FZ]

I will try it when I start working on EmuGUI 2.1. If your solution works, I'll merge your pull request, otherwise, I'll discuss further action.