Documenting project structure, state management, and event hierarchy

[ondrej33]

As the project structure gets more and more complex, we should start making some kind of external project documentation (apart from existing code documentation). Apart from the project structure, this should include a high-level description of the internal class hierarchy used to track the state of the sketch editor (and the analysis in future). This could initially be just some readme, but it should include UML diagrams and a more systematic description in future.

Similarly to that, we should make a list of events (together with their hierarchy) and their description. This will help in the task of refactoring them, too. For each event, we should note at least the following:

• its name and purpose
• components of the state it can modify
• its corresponding reverse events or "opposite-direction" events
• events it is related to (in case of more complex event relationships)