Code improvements

First round of code improvements:

Creating a kind of code structure for a better overview. Plus, adding the Guideline Support Library to write better code, and adding spdlogger to get fast logging.

The next commits will incorporate these two libraries.

More code improvements will follow this pull request.

Dependent on: https://gitlab.rlp.net/resteine/openio-fuse/merge_requests/4

I didn't really change the code logic. But please test if everything still works the way it was.