[Assets] Lazy object loading

[PxlBuzzard]

Right now Dash loads all meshes, textures, and prefabs at launch. Dash should offer more granular loading (and unloading) of assets during runtime.

One method to do this would be to build a list of all the objects that are available to load without loading them, then as game scripts and scenes reference objects they are loaded into memory.

[ColdenCullen]

Requires #184.