Add comparison method to directory thing and sub-components

[LukasPoque]

It should be easy to compare two things or sub-components (value, link, etc.).
=> Override == operator and hashcode.

[LukasPoque]

Overriding == and hashcode is a bad idea on mutable objects like thing (https://dart-lang.github.io/linter/lints/avoid_equals_and_hash_code_on_mutable_classes.html).

[LukasPoque]

If we decide to make things immutable we should use https://pub.dev/packages/equatable instead of overriding the normal comparison methods.

[LukasPoque]

IMO we should make all data classes immutable. They should have a copyWith()-Method to modify their attributes. This allows us to compare and use them in collections and other data structures where only immutable objects should be stored.