Disable portable mode by default on Windows

[nadiaholmquist]

Makes melonDS on Windows behave like it does in regard to its config directory on other platforms by default. The config file will now go in C:\Users\<username>\AppData\Local\melonDS.

I've added a check for whether the config file already exists in the executable path for backwards compatibility so this doesn't break people's pre-existing setups.

The motivation for this is that a lot of users get confused by the default behavior, either from not knowing that melonDS needs to be stored in a writable directory, or from presumably file synchronization tools marking paths in e.g. the user Documents folder as read-only, and melonDS suddenly failing to write its config file. Local AppData should always be a safe path to write into, so this will lead to less surprising error-prone behavior for users.

Remember that if you do want a portable version anyway, you can make a portable directory next to the executable and it will be used instead of the AppData path.