Follow XDG Spec and Application Data on Mac/Linux.

Currently MonoGame will dump a "SavedGames" folder in the Home folder on Linux and Mac (sometimes `~/Documents/`?), if the game uses the StorageDevice.

This does not obey the specification of either OS, and the SavedGames folder is not particularly useful like it is on Windows.

For Linux, we follow the XDG specification, where user savedata goes in XDG_DATA_HOME, which is `~/.local/share/` by default.

For OSX, we use `~/Library/Application Support/` by default.

So, for instance, if your game name is "VigilantePsychologist", your Linux saves will be in `~/.local/share/VigilantePsychologist/`, and your OSX saves will be in `~/Library/Applcation Support/VigilantePsychologist/`.

There is a fallback in place, but by the time we reach those fallbacks, it's expected that the OS running this game is just about screwed. Not all distros specify an XDG_DATA_HOME, but every *nix should provide a HOME.