Add SystemToolsStatic class to group SystemTools's static variables

This adds a new private class SystemToolsStatic that incorporates all static variables of SystemTools.

There are two motivations for doing this:

1. It feels cleaner and safer, since there's only new/delete pair and types become private.
2. It's a step towards making the SystemTools file system methods thread safe by adding a std::mutex to SystemToolsStatic.