Changed transform_component to inheirit from transform for now.
Working on adding a container of components to entity.

- Experimenting with a unique_id class which gives a unique (unless billions of unique_ids are
  created) integer id. Different types can have their own set of unique_ids.
- Experimenting with having add_component construct component (instead of passing in an
  already created component).
Game works with entity owning container of components.

- Replaced static member of unique_id with static method.