Remove scene render callbacks in favor of moving all renderable objects into the scene struct

[Vagabond]

The scene already has a list of objects to render, which we use for many things (not all, yet, though). This allows us to avoid a lot of custom object render management (and eventually would allow us to move the rendering to a different thread).

However, GUI components are very custom per-scene and we don't have the equivalent of a scenegraph for GUI components. We should change that. My proposal is to attach a guiframe to the scene struct, that each individual scene can pack their UI elements into. This would then allow us to generically navigate UIs in an automated way (like we do with the network menu shortcuts), which would allow for automated testing of the game via scripts.

This would also allow us to entirely eliminate the _render calls for all the scenes, scenes would only be responsible for managing the GUIs and objects on screen, not rendering them.