Asset Browser: Show current file assets in other asset libraries if contained

If the current file is saved within an asset library, showing that asset
library in the Asset Browser will also display the assets from this current
file now. In fact, it's the latest state of the open file, including all
unsaved modifications.
These assets will show a little Blender icon in the preview image, which is our
usual icon for current file data.

Note that this means an important design change: The "Current File" asset
library isn't the only place to edit assets from anymore. From now on assets
from the current file can also be edited in the context of the full asset
library. See T90193 for more info.

Technical info:
Besides just including the assets from the current `Main`, this requires
partial clearing and reading of file-lists, so that asset operations (e.g.
removing an asset data-block) doesn't require a full reload of the asset
library.

Maniphest Task: https://developer.blender.org/T90193

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