So I have been looking around to see what cross-platform library would be best suited to add XDG support. It seems like the C++ filesystem library is the best option.

Advantages
* the path class makes it super easy to concatenate two paths together and also has a stem function to separate the name from the extension (useful for creating .sav files).
* It has a function called create_directory (self-explanatory).
* For the path class, you can return a C string using c_str() (needed for fopen).

Disadvantages
* This is a fairly new addition to the C++ ISO (c++17), so some compilers might not support this (or require the "experimental/" added).
* While the filesystem library works on both Windows (Visual Studio) and Linux (g++), Linux requires that you add the "-lstdc++fs" flag. I am not sure how to add that on cmake.
* I haven't tested the filesystem library on a Mac.

I also want to change how MelonDS organizes its files. Currently, it stores everything in the same location as the binary. I want to change that so the program stores certain files in a particular folder.

Here are the folders I have in mind:

* config
* bios
* saves

In terms of how this should be implemented in MelonDS, I was thinking of having three functions:

* Function 1: Returns the platform specific location of where the program files should be stored. For example, function1() returns $XDG_DATA_HOME.
* Function 2: Combine Function 1 + melonds folder + a relative path to the data it wants to access. For example, function2(dummyfile) returns $XDG_DATA_HOME/melonds/dummyfile.
* Function 3: Create the melonds, config, bios, and saves folders if they are not there already.