Create hooks to engine in the editor

[evktalo]

As a developer,
I want to create hooks to engine,
so that I can listen to engine's events (e.g. shapes cached, konva loaded, engine initialized) in the editor and respond to them accordingly.

One usecase is hit region initialization, where we currently have an ugly setTimeout-based hack (when the users clicks the start button) to increase the chances that konva has cached the shapes. With hooks, we could just listen for that event and initialize the hit regions exactly when the shapes have been cached.