Asset dependencies

[seanmiddleditch]

Enable a means for asset dependencies to be declared, including:

• Runtime dependencies that must be preloaded (to load A, first we must load B)
• Runtime dependencies that must be loaded in parallel (A isn't fully loaded until we've loaded B, but the load of B may start after A)
• Runtime dependencies that may be late-loaded (A will explicitly load B at some point; we must ensure that B is on-disk though)
• Runtime dependencies that can be lazy-loaded (A will explicitly load B only if it is available; we can download B in the background while A is in use)
• Tool dependencies that should only be loaded in the editor (A needs B in order to build, but B should never be distributed to users)