Maintain functionality even after blend file/library is moved

[gregzaal]

Right now we use asset.library_weak_reference to get the path and determine if a datablock is an asset or not. This is not entirely always reliable and can cause strange behavior especially when the blend file or library itself is relocated.

Perhaps there's a better way to keep track of our assets (e.g. custom property) and maintain the ability to do things like resolution switching even after the user has moved their library or when the current file was moved in the OS and relative paths are no longer correct.