[Detail] Dropped nested struct in logger.

Due to dropping global attributes in logger class there is no need for nested `attributes` class. Just unwrapped it.
[Planning] Another planning pack.
[Unit Testing] Tiny decomposition.
[Unit Testing] Dead code elimination.
[Unit Testing] Prettifying.
[Unit Testing] More refactoring.
[Unit Testing] Tiny reordering.
[API] Verbosity property review.

Verbosity threshold should always be specified, when setting verbosity to a logger, even with filtering function.