Allow Win32 to store its saves, config files, etc., elsewhere.
If installed.txt exists(or the executable directory is read-only), use the user's My Documents(XP) or Documents(7/8/8.1) directory to search for everything. If it doesn't, or we can't determine the directory for some reason, use the old behaviour.
If installed.txt has a path in it, use that instead of the detected Documents directory.
If the custom directory or the current directory is read-only, fall back on the Documents directory.
After that point(if Documents is somehow read-only), we'd be screwed anyway, so there's no need to bother checking it..
Clean up GetSysDirectories. File::GetExeDirectory is much cleaner.
Merge pull request #4070 from thedax/win32Portability

Win32: Make PPSSPP have a portable mode.