Document management for asset files

[aclysma]

Need to implement functionality similar to the DocumentManager that takes care of assets. Minimal functionality would include:

• Knowing if an asset has in-memory changes that are not on disk
• Knowing asset on disk was modified so we can reload
• Forcing assets/packages to fully load (i.e. add a reference count for purpose of editor wanting to use reference them)

The document manager currently does the following for files:

• Open
• Save
• Save All
• Close
• Close All
• Query if allow changes/Force allow changes
• Query checkout status/Checkout
• IsOpen
• New

We also want a plan for detecting changed assets and updating entities, or updating assets that depend on some other asset.

This is the bulk of the functionality required for the editor to manipulate raw assets.