Combine Q and QCombination into one object

At this moment filters implementation consist of 3 classes:

• QNode - abstract representation of a Q tree Node
• Q - filter
• QCombination, a combination of Qs

I find the current structure hard to work with, eg. I don't see a clear way to implement "contains" method, for checking if a filter is already in. Same with counting filter's length.

We could divide the logic between managing the tree graph and managing filters:

• Node, a single node in the tree graph, holding methods for managing the tree graph only
• Q, holding methods for managing filters within a tree graph, inheriting from Node

The structure will be more clear, easier to maintain, what's more we can easily expand it - eg. implementing "contains" and "len" methods, negation, ...

I can handle it, just let me know if that sounds reasonable for you.
User-wise, nothing changes, unless one was using QNode or QCombination directly (probably not a case)

No file or test is named; start by locating the implementations of QNode, Q, and QCombination and reviewing their public behavior. Define completion as a clearer Node/Q structure that preserves existing user-facing behavior, while accounting for possible direct users of QNode or QCombination.