Make it easier to integrate with GG-Entities

[GGAlanSmithee]

Working within the current architecture is quite a good experience, but there are some painpoints when accessing / updating data from "the outside".

Some examples where this might needed could be:

• integrating it in an editor
• Using a thrid-party renderer which has it's own display list
• Probably many more cases...

Version 2 saw some nice upgrades towards this, such as making it possible to register / get systems etc by a key, but there are probably many more things which could be improved upon. There are already a bunch of issues touching on these kind of problems: #55 #61 #64 #65 #70.