This repo is a school project for Design Patterns.
use 'dub' to compile and run the code.
Entities are stored in a Command. Each Command is saved in History.
The rendering of Entities is done through History. First are
all Commands rendered with a foreach-loop.
If an Entity is added to a Group, The Entity is unable to save itself
anymore. From that point, Group has the task to save itself, including
all it's members.