Win32: Decouple the config files and memstick directory from the binary.

In order to prep for an eventual installer(https://github.com/hrydgard/ppsspp/issues/4061), PPSSPP should do the following:
1. Check for a "portable.txt" file in its own directory.

2a. If portable.txt exists, use the same old memstick folder it's been using since the beginning, and use PSP/Config as its new location for Controls.ini and PPSSPP.ini(this can of course be changed if nobody likes that, but it makes sense to me to do it like that, for organisation, and simpler code).
2b. If portable.txt doesn't exist, determine the user's My Documents(on XP) or Documents(on Vista/7/8) directory, and create a new folder called PPSSPP there.

Once PPSSPP does the above, this happens next:
We create the PSP folder inside the memstick folder(from 2a) or the PPSSPP folder we made in 2b. Then create the following folders: Cheats, Config, SAVEDATA, PPSSPP_STATE, and GAME.