Implement automatic calling of updateable entities

[PranavBahuguna]

Currently, there are many entities (models, lights etc.) that have an update function intended to be called in the main game loop. What we want is the ability for any updateable entity to automatically have this function be called every update cycle rather than having to do it ourselves. This will likely require implementation of the following:

• Some kind of overarching Updateable interface (can tie in with a transform interface, similar to Unity's Monobehaviour).
• Some way for the Main class to store and call updateables.
• Some way for updateables to be added and removed on demand.

Consider using something like the Observer pattern to achieve this.